Absolute Beginners

This area is intended for everyone new to PHP. It opens with a series of informal, entertaining tutorials written by Vikram Vaswani, founder and CEO of Melonfire. These tutorials build on a previously-published 5-part series which has now been updated and extended to embrace PHP 5, making parts of it suitable for those of you who already have worked with PHP 4 in the past.

If you came here to learn about elementary PHP 4 or basic PHP 5, this is for you. Enjoy!

The Only Acronym You'll Ever Need

If you're new to Web development, you could be forgiven for thinking that it consists of no more than a mass of acronyms, each one more indecipherable than the last. ASP, CGI, SOAP, XML, HTTP - the list seems never-ending, and the sheer volume of information on each of these can discourage the most avid programmer. But before you put on your running shoes and flee, there's a little secret you should know. To put together a cutting-edge Web site, chock full of all the latest bells and whistles, there's only one acronym you really need to know:

PHP

Now, while you have almost certainly heard of PHP, you may not be aware of just how powerful the language is, and how much it can do for you. Today, PHP has the enviable position of being the only open-source server-side scripting language that's both fun and easy to learn. This is not just advertising: recent surveys show that more than 16,000,000 Web sites use PHP as a server side scripting language, and the language also tops the list of most popular Apache modules.

Why, you ask? The short answer: it's powerful, it's easy to use, and it's free. Extremely robust and scalable, PHP can be used for the most demanding of applications, and delivers excellent performance even at high loads. Built-in database support means that you can begin creating data-driven applications immediately, XML support makes it suitable for the new generation of XML-enabled applications, and the extensible architecture makes it easy for developers to use it as a framework to build their own custom modules. Toss in a great manual, a knowledgeable developer community and a really low price (can you spell f-r-e-e?) and you've got the makings of a winner!

My goal in this series of tutorials is very simple: I'll be teaching you the basics of using PHP, and showing you why I think it's the best possible tool for Web application development today. I'll be making no assumptions about your level of knowledge, other than that you can understand basic HTML and have a sense of humor. And before you ask... Yes, this series covers both PHP 4 and PHP 5, with new PHP 5 features flagged for easy reference.

Let's get going!

The Right Environment

PHP is typically used in combination with a Web server like Apache. Requests for PHP scripts are received by the Web server, and are handled by the PHP interpreter. The results obtained after execution are returned to the Web server, which takes care of transmitting them to the client browser. Within the PHP script itself, the sky's the limit - your script can perform calculations, process user input, interact with a database, read and write files... Basically, anything you can do with a regular programming language, you can do inside your PHP scripts.

From the above, it is clear that in order to begin using PHP, you need to have a proper development environment set up.

This series will focus on using PHP with the Apache Web server on Linux, but you can just as easily use PHP with Apache on Windows, UNIX and Mac OS. Detailed instructions on how to set up this development environment on each platform are available in the online manual, at http://www.php.net/manual/en/installation.php - or you can just download a copy of PHP 5 from http://www.php.net and read the installation instructions.

Go do that now, and come back when you've successfully installed and tested PHP.

Start Me Up

There's one essential concept that you need to get your mind around before we proceed further. Unlike CGI scripts, which require you to write code to output HTML, PHP lets you embed PHP code in regular HTML pages, and execute the embedded PHP code when the page is requested.

These embedded PHP commands are enclosed within special start and end tags, like this:

<?php

... PHP code ...

?>

Here's a simple example that demonstrates how PHP and HTML can be combined:

<html>
<head></head>
<body>

Agent: So who do you think you are, anyhow?
<br />

<?php
// print output
echo 'Neo: I am Neo, but my people call me The One.';
?>

</body>
</html>

Not quite your traditional "Hello, World" program... but then again, I always thought tradition was over-rated.

Save the above script to a location under your Web server document root, with a .php extension, and browse to it. You'll see something like this:

Look at the HTML source:

<html>
<head></head>
<body>

Agent: So who do you think you are, anyhow?
<br />
Neo: I am Neo, but my people call me The One.
</body>
</html>

What just happened? When you requested the script above, Apache intercepted your request and handed it off to PHP. PHP then parsed the script, executing the code between the <?php...?> marks and replacing it with the output of the code run. The result was then handed back to the server and transmitted to the client. Since the output contained valid HTML, the browser was able to render it for display to the user.

A close look at the script will reveal the basic syntactical rules of PHP. Every PHP statement ends in a semi-colon. This convention is identical to that used in Perl, and omitting the semi-colon is one of the most common mistakes newbies make. That said, it is interesting to note that a semi-colon is not needed to terminate the last line of a PHP block. The PHP closing tag includes a semi-colon, therefore the following is perfectly valid PHP code:

<?php

// print output
echo 'Neo: I am Neo, but my people call me The One.'

?>

It's also possible to add comments to your PHP code, as I've done in the example above. PHP supports both single-line and multi-line comment blocks:

<?php

// this is a single-line comment

/* and this is a
multi-line
comment */

?>

Blank lines within the PHP tags are ignored by the parser. Everything outside the tags is also ignored by the parser, and returned as-is. Only the code between the tags is read and executed.

A Case of Identity

Variables are the bread and butter of every programming language... and PHP has them too. A variable can be thought of as a programming construct used to store both numeric and non-numeric data; the contents of a variable can be altered during program execution. Finally, variables can be compared with each other, and you - the programmer - can write code that performs specific actions on the basis of this comparison.

PHP supports a number of different variable types: integers, floating point numbers, strings and arrays. In many languages, it's essential to specify the variable type before using it: for example, a variable may need to be specified as type integer or type array. Give PHP credit for a little intelligence, though: it automagically determines variable type by the context in which it is being used!

Every variable has a name. In PHP, a variable name is preceded by a dollar ($) symbol and must begin with a letter or underscore, optionally followed by more letters, numbers and/or underscores. For example, $popeye, $one and $INCOME are all valid PHP variable names, while $123 and $48hrs are invalid.

Note that variable names in PHP are case sensitive, so $me is different from $Me or $ME.

Here's a simple example that demonstrates PHP's variables:

<html>
<head></head>
<body>

Agent: So who do you think you are, anyhow?
<br />

<?php
// define variables
$name = 'Neo';
$rank = 'Anomaly';
$serialNumber = 1;

// print output
echo "Neo: I am <b>$name</b>, the <b>$rank</b>. You can call me by my serial number, <b>$serialNumber</b>.";
?>

</body>
</html>

Here, the variables $name, $rank and $serialNumber are first defined with string and numeric values, and then substituted in the echo() function call. The echo() function, along with the print() function, is commonly used to print data to the standard output device (here, the browser). Notice that I've included HTML tags within the call to echo(), and those have been rendered by the browser in its output. You can do this too. Really.

An Equal Music

To assign a value to a variable, you use the assignment operator: the = symbol. This is used to assign a value (the right side of the equation) to a variable (the left side). The value being assigned need not always be fixed; it could also be another variable, an expression, or even an expression involving other variables, as below:

<?php

$age = $dob + 15;

?>

Interestingly, you can also perform more than one assignment at a time. Consider the following example, which assigns three variables the same value simultaneously:

<?php

$angle1 = $angle2 = $angle3 = 60;

?>

Not My Type

Every language has different types of variable - and PHP is no exception. The language supports a wide variety of data types, including simple numeric, character, string and Boolean types, and more complex arrays and objects. Here's a quick list of the basic ones, with examples:

· Boolean: The simplest variable type in PHP, a Boolean variable, simply specifies a true or false value.

<?php

$auth = true;

?>

· Integer: An integer is a plain-vanilla whole number like 75, -95, 2000 or 1.

<?php

$age = 99;

?>

· Floating-point: A floating-point number is typically a fractional number such as 12.5 or 3.141592653589. Floating point numbers may be specified using either decimal or scientific notation.

<?php

$temperature = 56.89;

?>

· String: A string is a sequence of characters, like "hello" or "abracadabra". String values may be enclosed in either double quotes ("") or single quotes(''). (Quotation marks within the string itself can be "escaped" with a backslash (\) character.) String values enclosed in double quotes are automatically parsed for special characters and variable names; if these are found, they are replaced with the appropriate value. Here's an example:

<?php

$identity = 'James Bond';
$car = 'BMW';

// this would contain the string "James Bond drives a BMW"
$sentence = "$identity drives a $car";
echo $sentence;

?>

To learn more about PHP's data types, visit http://www.php.net/manual/en/language.types.php.

Market Value

If variables are the building blocks of a programming language, operators are the glue that let you build something useful with them. You've already seen one example of an operator - the assignment operator -, which lets you assign a value to a variable. Since PHP believes in spoiling you, it also comes with operators for arithmetic, string, comparison and logical operations.

A good way to get familiar with operators is to use them to perform arithmetic operations on variables, as in the following example:

<html>
<head>
</head>
<body>

<?php

// set quantity
$quantity = 1000;

// set original and current unit price
$origPrice = 100;
$currPrice = 25;

// calculate difference in price
$diffPrice = $currPrice - $origPrice;

// calculate percentage change in price
$diffPricePercent = (($currPrice - $origPrice) * 100)/$origPrice

?>

<table border="1" cellpadding="5" cellspacing="0">
<tr>
<td>Quantity</td>
<td>Cost price</td>
<td>Current price</td>
<td>Absolute change in price</td>
<td>Percent change in price</td>
</tr>
<tr>
<td><?php echo $quantity ?></td>
<td><?php echo $origPrice ?></td>
<td><?php echo $currPrice ?></td>
<td><?php echo $diffPrice ?></td>
<td><?php echo $diffPricePercent ?>%</td>
</tr>
</table>

</body>
</html>

Looks complex? Don't be afraid - it's actually pretty simple. The meat of the script is at the top, where I've set up variables for the unit cost and the quantity. Next, I've performed a bunch of calculations using PHP's various mathematical operators, and stored the results of those calculations in different variables. The rest of the script is related to the display of the resulting calculations in a neat table.

If you'd like, you can even perform an arithmetic operation simultaneously with an assignment, by using the two operators together. The two code snippets below are equivalent:

<?php

// this...
$a = 5;
$a = $a + 10;

// ... is the same as this
$a = 5;
$a += 10;

?>

If you don't believe me, try echoing them both.

Stringing Things Along

Why stop with numbers? PHP also allows you to add strings with the string concatenation operator, represented by a period (.). Take a look:

<?php

// set up some string variables
$a = 'the';
$b = 'games';
$c = 'begin';
$d = 'now';

// combine them using the concatenation operator
// this returns 'the games begin now<br />'
$statement = $a.' '.$b.' '.$c.' '.$d.'<br />';
print $statement;

// and this returns 'begin the games now!'
$command = $c.' '.$a.' '.$b.' '.$d.'!';
print $command;

?>

As before, you can concatenate and assign simultaneously, as below:

<?php

// define string
$str = 'the';

// add and assign
$str .= 'n';

// str now contains "then"
echo $str;

?>

To learn more about PHP's arithmetic and string operators, visit http://www.php.net/manual/en/language.operators.arithmetic.php and http://www.php.net/manual/en/language.operators.string.php.

That's about it for this tutorial. You now know all about the basic building blocks and glue of PHP - its variables and operators. In Part Two of this series, I'll be using these fundamental concepts to demonstrate PHP's powerful form processing capabilities.

Not What You Expected

In Part One of this series, I gave you a brief introduction to PHP, and how it fits into your Web application development environment. I also taught you the basics of PHP variables, and showed you how to add, multiply and concatenate them together.

Now that you know the basics, it's time to focus in on one of PHP's nicer features - its ability to automatically receive user input from a Web form and convert it into PHP variables. If you're used to writing Perl code to retrieve form values in your CGI scripts, PHP's simpler approach is going to make you weep with joy. So get that handkerchief out, and scroll on down.

Form...

Forms have always been one of quickest and easiest ways to add interactivity to your Web site. A form allows you to ask customers if they like your products, casual visitors for comments on your site, and pretty girls for their phone numbers. And PHP can simplify the task of processing the data generated from a Web-based form substantially, as this first example demonstrates. Let's call this one form.htm.

<html>
<head></head>
<body>
<form action="message.php" method="post">
Enter your message: <input type="text" name="msg" size="30">
<input type="submit" value="Send">
</form>
</body>
</html>

The critical line in this page is the <form> tag

<form action="message.php" method="post">
...
</form>

As you probably already know, the "action" attribute of the <form> tag specifies the name of the server-side script (message.php in this case) that will process the information entered into the form. The "method" attribute specifies how the information will be passed.

...And Function

Now for the other half of the puzzle: the message.php script. This script reads the data submitted by the user and "does something with it". Here is message.php:

<html>
<head></head>
<body>

<?php
// retrieve form data
$input = $_POST['msg'];
// use it
echo "You said: <i>$input</i>";
?>

</body>
</html>

When you enter some data into form.htm (let's say "Boo"), and submit it, the form processor message.php will read it and display it to you ("You said: Boo"). Thus, whenever a form is submitted to a PHP script, all variable-value pairs within that form automatically become available for use within the script, through a special PHP container variable: $_POST. You can then access the value of the form variable by using its "name" inside the $_POST container, as I did in the script above.

Obviously, PHP also supports the GET method of form submission. All you need to do is change the "method" attribute to "get", and retrieve values from $_GET instead of $_POST. The $_GET and $_POST variables are actually a special type of PHP animal called an array, which I'll be teaching you about shortly. Don't worry too much about it at the moment, just make sure you're comfortable with retrieving simple values from a form with PHP, and then scroll on down to learn about some more operators that are useful in this context.

Operating With Extreme Caution

Thus far, the scripts we've discussed have been pretty dumb. All they've done is add numbers and strings, and read back to you the data you typed in yourself - not exactly overwhelming. In order to add some intelligence to your scripts, you need to know how to construct what geeks call a "conditional statement" - a statement which lets your script perform one of a series of possible actions based on the result of a comparison test. And since the basis of a conditional statement is comparison, you first need to know how to compare two variables and determine whether they're identical or different.

You've already seen some of PHP's arithmetic and string operators. However, the language also comes with operators designed specifically to compare two values: the so-called "comparison operators". Here's an example that demonstrates them in action:

<?php

/* define some variables */
$mean = 9;
$median = 10;
$mode = 9;

// less-than operator
// returns true if left side is less than right
// returns true here
$result = ($mean < $median);
print "result is $result<br />";

// greater-than operator
// returns true if left side is greater than right
// returns false here
$result = ($mean > $median);
print "result is $result<br />";

// less-than-or-equal-to operator
// returns true if left side is less than or equal to right
// returns false here
$result = ($median <= $mode);
print "result is $result<br />";

// greater-than-or-equal-to operator
// returns true if left side is greater than or equal to right
// returns true here
$result = ($median >= $mode);
print "result is $result<br />";

// equality operator
// returns true if left side is equal to right
// returns true here
$result = ($mean == $mode);
print "result is $result<br />";

// not-equal-to operator
// returns true if left side is not equal to right
// returns false here
$result = ($mean != $mode);
print "result is $result<br />";

// inequality operator
// returns true if left side is not equal to right
// returns false here
$result = ($mean <> $mode);
print "result is $result";

?>

The result of a comparison test is always Boolean: either true (1) or false (0 - which doesn't print anything). This makes comparison operators an indispensable part of your toolkit, as you can use them in combination with a conditional statement to send a script down any of its multiple action paths.

PHP 4.0 also introduced a new comparison operator, which allows you to test both for equality and type: the === operator. The following example demonstrates it:

<?php

/* define two variables */
$str = '10';
$int = 10;

/* returns true, since both variables contain the same value */
$result = ($str == $int);
print "result is $result<br />";

/* returns false, since the variables are not of the same type even though they have the same value */
$result = ($str === $int);
print "result is $result<br />";

/* returns true, since the variables are the same type and value */
$anotherInt = 10;
$result = ($anotherInt === $int);
print "result is $result";

?>

Read more about PHP's comparison operators at http://www.php.net/manual/en/language.operators.comparison.php.

A Question of Logic

In addition to the comparison operators I used so liberally above, PHP also provides four logical operators, which are designed to group conditional expressions together. These four operators - logical AND, logical OR, logical XOR and logical NOT - are illustrated in the following example::

<?php

/* define some variables */
$auth = 1;
$status = 1;
$role = 4;

/* logical AND returns true if all conditions are true */
// returns true
$result = (($auth == 1) && ($status != 0));
print "result is $result<br />";

/* logical OR returns true if any condition is true */
// returns true
$result = (($status == 1) || ($role <= 2));
print "result is $result<br />";

/* logical NOT returns true if the condition is false and vice-versa */
// returns false
$result = !($status == 1);
print "result is $result<br />";

/* logical XOR returns true if either of two conditions are true, or returns false if both conditions are true */
// returns false
$result = (($status == 1) xor ($auth == 1));
print "result is $result<br />";

?>

Logical operators play an important role in building conditional statements, as they can be used to link together related conditions simply and elegantly. View more examples of how they can be used at http://www.php.net/manual/en/language.operators.logical.php.

Older But Not Wiser

Now that you've learnt all about comparison and logical operators, I can teach you about conditional statements. As noted earlier, a conditional statement allows you to test whether a specific condition is true or false, and perform different actions on the basis of the result. In PHP, the simplest form of conditional statement is the if() statement, which looks something like this:

if (condition) {
    do this!
}

The argument to if()is a conditional expression, which evaluates to either true or false. If the statement evaluates to true, all PHP code within the curly braces is executed; if it does not, the code within the curly braces is skipped and the lines following the if() construct are executed.

Let me show you how the if() statement works by combining it with a form. In this example, the user is asked to enter his or her age.

<html>
<head></head>
<body>
<form action="ageist.php" method="post">
Enter your age: <input name="age" size="2">
</form>
</body>
</html>

Depending on whether the entered age is above or below 21, a different message is displayed by the ageist.php script:

<html>
<head></head>
<body>

<?php
// retrieve form data
$age = $_POST['age'];
// check entered value and branch
if ($age >= 21) {
     echo 'Come on in, we have alcohol and music awaiting you!';
}
if ($age < 21) {
     echo 'You\'re too young for this club, come back when you\'re a little older';
}
?>

</body>
</html>

If Not This, Then What?

In addition to the if() statement, PHP also offers the if-else construct, used to define a block of code that gets executed when the conditional expression in the if() statement evaluates as false.

The if-else construct looks like this:

if (condition) {
    do this!
    }
else {
    do this!
}

This construct can be used to great effect in the last example: we can combine the two separate if()statements into a single if-else statement.

<html>
<head></head>
<body>

<?php
// retrieve form data
$age = $_POST['age'];
// check entered value and branch
if ($age >= 21) {
    echo 'Come on in, we have alcohol and music awaiting you!';
    }
else {
    echo 'You\'re too young for this club, come back when you\'re a little older';
}
?>

</body>
</html>

Spreading Confusion

If the thought of confusing people who read your code makes you feel warm and tingly, you're going to love the ternary operator, represented by a question mark (?). This operator, which lets you make your conditional statements almost unintelligible, provides shortcut syntax for creating a single-statement if-else block. So, while you could do this:

<?php

if ($numTries > 10) {
$msg = 'Blocking your account...';
    }
else {
$msg = 'Welcome!';
}

?>

You could also do this, which is equivalent (and a lot more fun):

<?php

$msg = $numTries > 10 ? 'Blocking your account...' : 'Welcome!';

?>

PHP also lets you "nest" conditional statements inside each other. For example, this is perfectly valid PHP code:

<?php

if ($day == 'Thursday') {
    if ($time == '0800') {
        if ($country == 'UK') {
$meal = 'bacon and eggs';
        }
    }
}

?>

Another, more elegant way to write the above is with a series of logical operators:

<?php

if ($day == 'Thursday' && $time == '0800' && $country == 'UK') {
$meal = 'bacon and eggs';
}

?>

The Daily Special

PHP also provides you with a way of handling multiple possibilities: the if-elseif-else construct. A typical if-elseif-else statement block would look like this:

if (first condition is true) {
    do this!
    }
elseif (second condition is true) {
    do this!
    }
elseif (third condition is true) {
    do this!
    }
  ... and so on ...
else {
     do this!
}

And here's an example that demonstrates how to use it:

<html>
<head></head>
<body>
<h2>Today's Special</h2>
<p>
<form method="get" action="cooking.php">
<select name="day">
<option value="1">Monday/Wednesday
<option value="2">Tuesday/Thursday
<option value="3">Friday/Sunday
<option value="4">Saturday
</select>
<input type="submit" value="Send">
</form>
</body>
</html>

As you can see, this is simply a form which allows you to pick a day of the week. The real work is done by the PHP script cooking.php:

<html>
<head></head>
<body>

<?php
// get form selection
$day = $_GET['day'];
// check value and select appropriate item
if ($day == 1) {
$special = 'Chicken in oyster sauce';
    }
elseif ($day == 2) {
$special = 'French onion soup';
    }
elseif ($day == 3) {
$special = 'Pork chops with mashed potatoes and green salad';
    }
else {
$special = 'Fish and chips';
}
?>

<h2>Today's special is:</h2>
<?php echo $special; ?>
</body>
</html>

In this case, I've used the if-elseif-else control structure to assign a different menu special to each combination of days. Note that as soon as one of the if() branches within the block is found to be true, PHP will execute the corresponding code, skip the remaining if() statements in the block, and jump immediately to the lines following the entire if-elseif-else block.

And that's it for now. To view more examples of conditional statements in action, visit http://www.php.net/manual/en/language.control-structures.php. In Part Three, I'll be bringing you more control structures, more operators and more strange and wacky scripts - so make sure you don't miss it!

Going Deeper

If you've been paying attention, you remember that in Part Two I gave you a quick crash course in PHP's basic control structures and operators. I also showed you how PHP can be used to process the data entered into a Web form. In this tutorial, I'm going to delve deeper into PHP's operators and control structures, showing you two new operators, an alternative to the if-else() family of conditional statements, and some of PHP's more interesting loops. So keep reading... this is just about to get interesting!

Switching Things Around

An alternative to the if-else() family of control structures is PHP's switch-case() statement, which does almost the same thing. It looks like this:

switch (decision-variable) {
    case first condition is true:
        do this!
    case second condition is true:
        do this!
      ... and so on...
}

Depending on the value of the decision variable, the appropriate case() block is executed. A default block can also be created, to handle all those occasions when the value of the decision variable does not match any of the listed case() conditions.

I'll make this a little clearer by re-writing one of my earlier examples in terms of the switch() statement:

<html>
<head></head>
<body>

<?php

// get form selection
$day = $_GET['day'];
// check value and select appropriate item
switch ($day) {
    case 1:
$special = 'Chicken in oyster sauce';
        break;
    case 2:
$special = 'French onion soup';
        break;
    case 3:
$special = 'Pork chops with mashed potatoes and green salad';
        break;
    default:
$special = 'Fish and chips';
        break;
}

?>

<h2>Today's special is:</h2>
<?php echo $special ?>
</body>
</html>

There are a couple of important keywords here:

· The break keyword is used to break out of the switch() statement block and move immediately to the lines following it.

· The default keyword is used to execute a default set of statements when the variable passed to switch() does not satisfy any of the conditions listed within the block.

A common newbie mistake here is to forget the break at the end of every case() block. Remember that if you forget to break out of a case() block, PHP will continue executing the code in all the subsequent case() blocks it encounters.

For more on the switch() statement, see http://www.php.net/manual/en/control-structures.switch.php.

Creative Conditionals

Normally, when creating and processing forms in PHP, you would place the HTML form in one file, and handle form processing through a separate PHP script. However, with the power of conditional statements at your disposal, you can combine both pages into one.

How do you do this? Simple. All you need to do is assign a name to the form submit control, and then check whether the special $_POST container variable contains that name when the script first loads up. If it does, the form has already been submitted, and you can process the data; if it does not, that the user has not submitted the form and you therefore need to generate the initial, unfilled form. Thus, by testing for the presence or absence of this submit variable, a clever PHP programmer can use a single PHP script to generate both the initial form, and the output after it has been submitted, as appropriate.

Here's a simple example:

<html>
<head></head>
<body>

<?php
/* if the "submit" variable does not exist, the form has not been submitted - display initial page */
if (!isset($_POST['submit'])) {
?>

    <form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="post">
    Enter your age: <input name="age" size="2">
    <input type="submit" name="submit" value="Go">
    </form>

<?php
    }
else {
/* if the "submit" variable exists, the form has been submitted - look for and process form data */
    // display result
$age = $_POST['age'];
    if ($age >= 21) {
        echo 'Come on in, we have alcohol and music awaiting you!';
        }
    else {
        echo 'You\'re too young for this club, come back when you\'re a little older';
    }
}
?>

</body>
</html>

As you can see, the script contains two pages: the initial, empty form and the result page generated after hitting the submit button. In order to decide which page to display, the script first tests for the presence of the $_POST['submit'] variable. If it doesn't find it, it assumes that the form has yet to be submitted, and displays the initial list of days. Once the form has been submitted, the same script will be called to process the form input. This time, however, the $_POST['submit'] variable will be set, and so PHP will not display the initial page, but rather the page containing the result message.

Note that for this to work, your submit button must have a value assigned to its "name" attribute, and you must check for that value in the primary conditional statement. And in case you were wondering, the $_SERVER array is a special PHP variable which always holds server information, including the path and name of the currently executing script.

Next up, loops.

One by One

For those of you unfamiliar with the term, a loop is a control structure that enables you to repeat the same set of php
statements or commands over and over again (the actual number of repetitions can be a number you specify, or depend on the fulfillment of one or more conditions).

Now, last time out you saw a few comparison and logical operators, which help in building conditional statements. Since this segment of the tutorial is going to focus on loops, this is an appropriate time to introduce you to PHP's auto-increment and auto-decrement operators, which see a lot of use in this context.

The auto-increment operator is a PHP operator designed to automatically increment the value of the variable it is attached to by 1. It is represented by two "plus" signs (++). This snippet of code should explain it:

<?php

// define $total as 10
$total = 10;
// increment it
$total++;
// $total is now 11
echo $total;

?>

Thus, $total++ is functionally equivalent to $total = $total + 1.

There's a corresponding auto-decrement operator (--), which does exactly the opposite:

<?php

// define $total as 10
$total = 10;
// decrement it
$total--;
// $total is now 9
echo $total;

?>

These operators are frequently used in loops, to update the value of the loop counter, speaking of which...

Being Square

The first - and simplest - loop to learn in PHP is the so-called while() loop, which looks like this:

while (condition is true) {
    do this!
}

In this case, so long as the condition specified evaluates as true - remember what you learned in Part Two? - the PHP statements within the curly braces will continue to execute. As soon as the condition becomes false, the loop will be broken and the statements following it will be executed.

Here's a quick example which demonstrates the while() loop:

<html>
<head></head>
<body>
<form action="squares.php" method="POST">
Print all the squares between 1 and <input type="text" name="limit" size="4" maxlength="4">
<input type="submit" name="submit" value="Go">
</form>
</body>
</html>

This is a simple form which asks the user to enter a number. When the form is submitted, the PHP script that is invoked should take this number and print the squares of all the numbers between 1 and the entered value. With a while() loop, this is simplicity itself:

<html>
<head></head>
<body>

<?php

// set variables from form input
$upperLimit = $_POST['limit'];
$lowerLimit = 1;
// keep printing squares until lower limit = upper limit
while ($lowerLimit <= $upperLimit) {
    echo ($lowerLimit * $lowerLimit).'&nbsp;';
$lowerLimit++;
}
// print end marker
echo 'END';

?>

</body>
</html>

This script uses a while() loop to count forwards from 1 until the values of $lowerLimit and $upperLimit are equal.

Loop First, Ask Questions Later

The while() loop executes a set of statements while a specified condition is true. But what happens if the condition is true on the first iteration of the loop itself? In the previous example, if you were to enter the value 0in the form, the while() loop would not execute even once. Try it yourself and you'll see what I mean.

If you're in a situation where you need to execute a set of statements *at least* once, PHP offers you the do-while() loop. Here's what it looks like:

do {
    do this!
} while (condition is true)

Let's take a quick example to better understand the difference between while() and do-while():

<?php

$x = 100;
// while loop
while ($x == 700) {
    echo "Running...";
    break;
}

?>

In this case, no matter how many times you run this PHP script, you will get no output at all, since the value of $x is not equal to 700. But, if you ran this version of the script:

<?php

$x = 100;
// do-while loop
do {
    echo "Running...";
    break;
} while ($x == 700);

?>

you would see one line of output, as the code within the do() block would run once.

Let's now revise the previous PHP script so that it runs at least once, regardless of what value is entered into the form:

<html>
<head></head>
<body>

<?php

// set variables from form input
$upperLimit = $_POST['limit'];
$lowerLimit = 1;
// keep printing squares until lower limit = upper limit
do {
    echo ($lowerLimit * $lowerLimit).'&nbsp;';
$lowerLimit++;
} while ($lowerLimit <= $upperLimit);
// print end marker
echo ' END';

?>

</body>
</html>

Thus, the construction of the do-while() loop is such that the statements within the loop are executed first, and the condition to be tested is checked afterwards. This implies that the statements within the curly braces would be executed at least once.

Read more about the while() and do-while() loops at http://www.php.net/manual/en/control-structures.while.php and http://www.php.net/manual/en/control-structures.do.while.php.

Doing it by Numbers

Both the while() and do-while() loops continue to iterate for as long as the specified conditional expression remains true. But what if you need to execute a certain set of statements a specific number of times - for example, printing a series of thirteen sequential numbers, or repeating a particular set of <td> cells five times? In such cases, clever programmers reach for the for() loop...

The for() loop typically looks like this:

for (initial value of counter; condition; new value of counter) {
    do this!
}

Looks like gibberish? Well, hang in there for a minute...the "counter" here is a PHP variable that is initialized to a numeric value, and keeps track of the number of times the loop is executed. Before each execution of the loop, the "condition" is tested. If it evaluates to true, the loop will execute once more and the counter will be appropriately incremented; if it evaluates to false, the loop will be broken and the lines following it will be executed instead.

Here's a simple example that demonstrates how this loop can be used:

<html>
<head>
<basefont face="Arial">
</head>
<body>

<?php

// define the number
$number = 13;
// use a for loop to calculate tables for that number
for ($x = 1; $x <= 10; $x++) {
    echo "$number x $x = ".($number * $x)."<br />";
}

?>

</body>
</html>

The first thing I've done here is define the number to be used for the multiplication table. I've used 13 here - for no reason other than that it rhymes with "green".

Next, I've constructed a for() loop with $x as the counter variable, initialized it to 1. and specified that the loop should run no more than 10 times. The auto-increment operator (discussed earlier) automatically increments the counter by 1 every time the loop is executed. Within the loop, the counter is multiplied by the number, to create the multiplication table, and echo() is used to display the result on the page.

Turning the Tables

As you just saw, a for() loop is a very interesting - and useful - programming construct. The next example illustrates its usefulness in a manner that should endear it to any HTML programmer.

<html>
<head></head>
<body>
<form method="post" action="<?php echo $_SERVER['PHP_SELF']; ?>">
Enter number of rows <input name="rows" type="text" size="4"> and columns <input name="columns" type="text" size="4"> <input type="submit" name="submit" value="Draw Table">
</form>

<?php

if (isset($_POST['submit'])) {
    echo "<table width = 90% border = '1' cellspacing = '5' cellpadding = '0'>";
// set variables from form input
$rows = $_POST['rows'];
$columns = $_POST['columns'];
// loop to create rows
for ($r = 1; $r <= $rows; $r++) {
        echo "<tr>";
// loop to create columns
for ($c = 1; $c <= $columns;$c++) {
            echo "<td>&nbsp;</td>\n";
        }     echo "</tr>\n";
    }
    echo "</table>\n";
}

?>

</body>
</html>

As you'll see if you try coding the same thing by hand, PHP's for() loop just saved you a whole lot of work! And it looks good too - take a look at the source code of the dynamically generated table, and you'll see that it's nicely formatted, with line breaks at the end of every table cell and row. This magic is accomplished by forcing a carriage return with \n in every call to echo().

For more examples of the for() loop in action, visit http://www.php.net/manual/en/control-structures.for.php.

Loops are frequently used in combination with one of PHP's more complex data types, the animal known as the array. That's a whole topic in itself, and in fact I'm going to discuss it in detail in the next segment of this tutorial. Then I'm going to show you how arrays, loops and forms all work together to make the creation of complex Web forms as easy as eating pie. All that and more in Part Four!

A Big Mistake

Having spent lots of time travelling around the outer landscape of PHP  - learning all about control structures, operators and variables  - you're probably bored. You might even be thinking of dropping out right now, and instead spending your time more constructively (or so you think) in front of the idiot box.

That would be a big mistake. And when I say big, I mean humongous.

You see, if you forego this segment of the tutorial for the dubious charms of Ally McBeal, you're going to miss out on one of PHP's coolest variable types. It's a little thing called an array, and I'm not exaggerating when I tell you that once you're on speaking terms with it, you're never going to look at a PHP script the same way again. But hey, don't take my word for it... toss that remote aside and come see for yourself!

Fruity Pizza

Thus far, the variables we've discussed contained only a single value, such as:

<?php

$i = 5;

?>

However, array variables are a different kettle of fish altogether. An array is a complex variable that allows you to store multiple values in a single variable (which is handy when you need to store and represent related information). Think of the array variable as a "container" variable, which can contain one or more values. For example:

<?php

// define an array
$pizzaToppings = array('onion', 'tomato', 'cheese', 'anchovies', 'ham', 'pepperoni');
print_r($pizzaToppings);

?>

Here, $pizzaToppings is an array variable, which contains the values 'onion', 'tomato', 'cheese', 'anchovies', 'ham' and 'pepperoni'. (Array variables are particularly useful for grouping related values together.)

print_r() is a special function that allows you to take a sneak peek inside an array. It's more useful for debugging (finding out why your script doesn't work) than it is for display purposes, but I'll use it here so you can see what's going on under the surface. You do have your server running and your browser open, right?

The various elements of the array are accessed via an index number, with the first element starting at zero. So, to access the element 'onion', you would use the notation $pizzaToppings[0], while 'anchovies' would be $pizzaToppings[3] - essentially, the array variable name followed by the index number enclosed within square braces.

PHP also allows you to replace indices with user-defined "keys", in order to create a slightly different type of array. Each key is unique, and corresponds to a single value within the array.

<?php

// define an array
$fruits = array('red' => 'apple', 'yellow' => 'banana', 'purple' => 'plum', 'green' => 'grape');
print_r($fruits);

?>

In this case, $fruits is an array variable containing four key-value pairs. (The => symbol is used to indicate the association between a key and its value.) In order to access the value 'banana', you would use the notation $fruits['yellow'], while the value 'grape' would be accessible via the notation $fruits['green'].

This type of array is sometimes referred to as a "hash" or "associative array". If you've ever used Perl, you'll see the similarities to the Perl hash variable.

Eating Italian

The simplest was to define an array variable is the array() function. Here's how:

<?php

// define an array
$pasta = array('spaghetti', 'penne', 'macaroni');

?>

The rules for choosing an array variable name are the same as those for any other PHP variable: it must begin with a letter or underscore, and can optionally be followed by more letters, numbers and underscores.

Alternatively, you can define an array by specifying values for each element in the index notation, like this:

<?php

// define an array
$pasta[0] = 'spaghetti';
$pasta[1] = 'penne';
$pasta[2] = 'macaroni';

?>

If you're someone who prefers to use keys rather than default numeric indices, you might prefer the following example:

<?php

// define an array
$menu['breakfast'] = 'bacon and eggs';
$menu['lunch'] = 'roast beef';
$menu['dinner'] = 'lasagna';

?>

You can add elements to the array in a similar manner. For example, if you wanted to add the element 'green olives' to the $pizzaToppings array, you would use something like this:

<?php

// add an element to an array
$pizzaToppings[3] = 'green olives';

?>

In order to modify an element of an array, simply assign a new value to the corresponding scalar variable. If you wanted to replace 'ham' with 'chicken', you'd use:

<?php

// modify an array
$pizzaToppings[4] = 'chicken';

?>

You can do the same using keys. The following statement modifies the element with the key 'lunch' to a different value:

<?php

// modify an array
$menu['lunch'] = 'steak with mashed potatoes';

?>

Push And Pull

You can also add an element to the end of an existing array with the array_push() function:

<?php

// define an array
$pasta = array('spaghetti', 'penne', 'macaroni');

// add an element to the end
array_push($pasta, 'tagliatelle');

print_r($pasta);

?>

And you can remove an element from the end of an array using the interestingly-named array_pop() function.

<?php

// define an array
$pasta = array('spaghetti', 'penne', 'macaroni');

// remove an element from the end
array_pop($pasta);

print_r($pasta);

?>

If you need to pop an element off the top of the array, you can use the array_shift() function:

<?php

// define an array
$pasta = array('spaghetti', 'penne', 'macaroni');

// take an element off the top
array_shift($pasta);

print_r($pasta);

?>

And the array_unshift() function takes care of adding elements to the beginning of the array.

<?php

// define an array
$pasta = array('spaghetti', 'penne', 'macaroni');

// add an element to the beginning
array_unshift($pasta, 'tagliatelle');

print_r($pasta);

?>

The explode() function splits a string into smaller components, based on a user-specified delimiter, and returns the pieces as elements as an array.

<?php

// define CSV string
$str = 'red, blue, green, yellow';

// split into individual words
$colors = explode(', ', $str);

print_r($colors);

?>

To do the reverse, you can use the implode() function, which creates a single string from all the elements of an array by joining them together with a user-defined delimiter. Reversing the example above, we have:

<?php

// define array
$colors = array ('red', 'blue', 'green', 'yellow');

// join into single string with 'and'
// returns 'red and blue and green and yellow'
$str = implode(' and ', $colors);

print $str;

?>

Finally, the two examples below show how the sort() and rsort()functions can be used to sort an array alphabetically (or numerically), in ascending and descending order respectively:

<?php

// define an array
$pasta = array('spaghetti', 'penne', 'macaroni');

// returns the array sorted alphabetically
sort($pasta);
print_r($pasta);
print "<br />";

// returns the array sorted alphabetically in reverse
rsort($pasta);
print_r($pasta);

?>

Looping the Loop

So that takes care of putting data inside an array. Now, how about getting it out?

Retrieving data from an array is pretty simple: all you need to do is access the appropriate element of the array using its index number. To read an entire array you simply loop over it, using any of the loop constructs you learned about in Part Three of this tutorial.

How about a quick example?

<html>
<head></head>
<body>
My favourite bands are:
<ul>

<?php

// define array
$artists = array('Metallica', 'Evanescence', 'Linkin Park', 'Guns n Roses');
// loop over it and print array elements
for ($x = 0; $x < sizeof($artists); $x++) {
    echo '<li>'.$artists[$x];
}

?>

</ul>
</body>
</html>

When you run this script, here's what you'll see:

My favourite bands are:

· Metallica

· Evanescence

· Linkin Park

· Guns n Roses

In this case, I've defined an array, and then used the for() loop to: run through it, extract the elements using the index notation, and display them one after the other.

I'll draw your attention here to the sizeof() function. This function is one of the most important and commonly used array functions. It returns the size of (read: number of elements within) the array. It is mostly used in loop counters to ensure that the loop iterates as many times as there are elements in the array.

If you're using an associative array, the array_keys() and array_values()functions come in handy, to get a list of all the keys and values within the array.

<?php

// define an array
$menu = array('breakfast' => 'bacon and eggs', 'lunch' => 'roast beef', 'dinner' => 'lasagna');

/* returns the array ('breakfast', 'lunch', 'dinner') with numeric indices */
$result = array_keys($menu);
print_r($result);
print "<br />";

/* returns the array ('bacon and eggs', 'roast beef', 'lasagna') with numeric indices */
$result = array_values($menu);
print_r($result);

?>

What's That Noise?

There is, however, a simpler way of extracting all the elements of an array. PHP 4.0 introduced a spanking-new loop type designed specifically for the purpose of iterating over an array: the foreach() loop. (It is similar in syntax to the Perl construct of the same name.) Here's what it looks like:

foreach ($array as $temp) {
    do this!
}

A foreach() loop runs once for each element of the array passed to it as argument, moving forward through the array on each iteration. Unlike a for() loop, it doesn't need a counter or a call to sizeof(), because it keeps track of its position in the array automatically. On each run, the statements within the curly braces are executed, and the currently-selected array element is made available through a temporary loop variable.

To better understand how this works, consider this rewrite of the previous example, using the foreach() loop:

<html>
<head></head>
<body>
My favourite bands are:
<ul>

<?php

// define array
$artists = array('Metallica', 'Evanescence', 'Linkin Park', 'Guns n Roses');
// loop over it
// print array elements
foreach ($artists as $a) {
    echo '<li>'.$a;
}

?>

</ul>
</body>
</html>

Each time the loop executes, it places the currently-selected array element in the temporary variable $a. This variable can then be used by the statements inside the loop block. Since a foreach() loop doesn't need a counter to keep track of where it is in the array, it is lower-maintenance and also much easier to read than a standard for() loop. Oh yeah... and it also works with associative arrays, with no extra programming needed.

Music for the Masses

In addition to their obvious uses, arrays and loops also come in handy when processing forms in PHP. For example, if you have a group of related checkboxes or a multi-select list, you can use an array to capture all the selected form values in a single variable, to simplify processing. Consider the following example, which illustrates this:

<html>
<head></head>
<body>

<?php
// check for submit
if (!isset($_POST['submit'])) {
// and display form
?>

    <form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="POST">
    <input type="checkbox" name="artist[]" value="Bon Jovi">Bon Jovi
    <input type="checkbox" name="artist[]" value="N'Sync">N'Sync
    <input type="checkbox" name="artist[]" value="Boyzone">Boyzone
    <input type="checkbox" name="artist[]" value="Britney Spears">Britney Spears
    <input type="checkbox" name="artist[]" value="Jethro Tull">Jethro Tull
    <input type="checkbox" name="artist[]" value="Crosby, Stills & Nash">Crosby, Stills & Nash
    <input type="submit" name="submit" value="Select">
    </form>

<?php
}
else {
// or display the selected artists
    // use a foreach loop to read and display array elements
if (is_array($_POST['artist'])) {
        echo 'You selected: <br />';
        foreach ($_POST['artist'] as $a) {
           echo "<i>$a</i><br />";
            }
        }
    else {
        echo 'Nothing selected';
    }
}
?>

</body>
</html>

When the above form is submitted, PHP will automatically create an array variable, and populate it with the items selected. This array can then be processed with a foreach() loop, and the selected items retrieved from it.

You can do this with a multi-select list also, simply by using array notation in the select control's "name" attribute. Try it out for yourself and see... and make sure you tune in for the next PHP 101 tutorial, same time, same channel.

Back to School

When you first started reading this series, I promised you that you'd have a whole lot of fun. If you're the cynical type, you may be feeling that I didn't keep my promise. After all, how much fun have you really had so far? All you've done is learn a bunch of theoretical rules, added and subtracted numbers from each other, learnt primitive decision-making and gone round and round in the circular funhouse of loops. Heck, if this wasn't a PHP tutorial, it would be kindergarten...

I hear you.

In this segment of our ongoing saga, I'm going to teach you how to do something that's definitely not for kids. It involves getting down and dirty with files on the disk: meeting them (shock!), reading their contents (shriek!) and (horror of horrors!) writing data to them. All of these exciting activities will take place under the aegis of PHP's very cool file manipulation API, which allows you to view and modify file attributes, read and list directory contents, alter file permissions, retrieve file contents into a variety of native data structures, and search for files based on specific patterns.

Let's get started!

Handle With Care

I'll begin with something simple: opening a file and reading its contents. Let's assume that somewhere on your disk, hidden under /usr/local/stuff/that/should/be/elsewhere/recipes/, you have a text file containing the recipe for the perfect Spanish omelette. You now wish to read the contents of this file into a PHP script.

In order to do this, there are three distinct steps to be followed:

· Open the file and assign it a file handle.

· Interact with the file, via its handle, and extract its contents into a PHP variable.

· Close the file.

Here's a PHP script that does just that:

<?php

// set file to read
$file = '/usr/local/stuff/that/should/be/elsewhere/recipes/omelette.txt';
// open file
$fh = fopen($file, 'r') or die('Could not open file!');
// read file contents
$data = fread($fh, filesize($file)) or die('Could not read file!');
// close file
fclose($fh);
// print file contents
echo $data;

?>

Run this script through your Web browser, and PHP should return the contents of the file.

Now let me explain each of the three steps above in detail:

Open the file and assign it a file handle

PHP needs a file handle to read data from a file. This file handle can be created with the fopen() function, which accepts two arguments: the name and path to the file, and a string indicating the "mode" in which the file is to be opened ('r' for read).

Three different modes are available for use with the fopen() function. Here's the list:

'r' - opens a file in read mode

'w' - opens a file in write mode, destroying existing file contents

'a' - opens a file in append mode, preserving existing file contents

Interact with the file via its handle and extract its contents into a PHP variable

If the fopen() function is successful, it returns a file handle, $fh, which can be used for further interaction with the file. This file handle is used by the fread() function, which reads the file and places its contents into a variable.

The second argument to fread() is the number of bytes to be read. You can usually obtain this information through the filesize() function, which - who'd have guessed it?!- returns the size of the file in bytes.

Close the file

This last step is not strictly necessary as PHP closes the file automatically once it reaches the end of the script, but it's a good habit to develop. Explicitly closing the file with fclose() has two advantages: it ties up loose ends in your script, and it wins you lots of good karma from the PHP community.

You probably haven't see the die() function before, either. This function is mostly used as a primitive error-handling mechanism. In the event of a fatal error, such as the file path being invalid or the file permissions being such that PHP cannot read it, die() terminates script processing and optionally displays a user-specified error message indicating why it committed suicide.

Different Strokes

An alternative method of reading data from a file is the very cool file() function, which reads the entire file into an array (remember them?) with one line of code. Each element of the array then contains one line from the file. To display the contents of the file, simply iterate over the array in a foreach() loop and print each element.

The following example demonstrates:

<?php

// set file to read
$file = '/usr/local/stuff/that/should/be/elsewhere/recipes/omelette.txt';
// read file into array
$data = file($file) or die('Could not read file!');
// loop through array and print each line
foreach ($data as $line) {
     echo $line;
}

?>

In this example, the file() command opens the file, reads it into an array and closes the file - all in one, single, elegant movement. Each element of the array now corresponds to a line from the file. It's easy to print the file's contents now - just reach for that mainstay of array processing, the foreach() loop.

Don't want the data in an array? Try the file_get_contents() function, new in PHP 4.3.0 and PHP 5.0, which reads the entire file into a string:

<?php

// set file to read
$file = '/usr/local/stuff/that/should/be/elsewhere/recipes/omelette.txt';
// read file into string
$data = file_get_contents($file) or die('Could not read file!');
// print contents
echo $data;

?>

Who am I kidding? I always use the one-line functions noted above instead of the three-line sequence of fopen(), fread() and fclose(). Laziness conquers all.

When Laziness is a Virtue

PHP also offers two very useful functions to import files into a PHP script: the include() and require()functions. These functions can be used to suck external files lock, stock and barrel into a PHP script, which is very handy if, for example, you have a modular application which has its code broken down across files in separate locations.

The best way to understand the utility of the include() and require() functions is with an example. Assume that on your Web site you have a standard menu bar at the top of every page, and a standard copyright notice in the bottom. Instead of copying and pasting the header and footer code on each individual page, PHP gurus simply create separate files for the header and footer, and import them at the top and bottom of each script. This also makes a change to the site design easier to implement: instead of manually editing a gazillion files, you simply edit two, and the changes are reflected across your entire site instantaneously.

Let's see a real live example of this in action. Create the header in one file, called header.php:

<html>
<head>
<title><?php echo $page['title'];?></title>
</head>
<body>
<!-- top menu bar -->
<table width="90%" border="0" cellspacing="5" cellpadding="5">
<tr>
<td><a href="#">Home</a></td>
<td><a href="#">Site Map</a></td>
<td><a href="#">Search</a></td>
<td><a href="#">Help</a></td>
</tr>
</table>
<!-- header ends -->

Next, create the footer with the copyright notice in a second file, footer.php:

<!-- footer begins -->
<br />
<center>Your usage of this site is subject to its published <a href="tac.html">terms and conditions</a>. Data is copyright Big Company Inc, 1995-<?php echo date("Y", mktime()); ?></center>
</body>
</html>

Finally, create a script to display the main content of your site, and include() the header and footer at appropriate places:

<?php

// create an array to set page-level variables
$page = array();
$page['title'] = 'Product Catalog';
/* once the file is imported, the variables set above will become available to it */

// include the page header
include('header.php');

?>

<!-- HTML content here -->

<?php

// include the page footer
include('footer.php');

?>

Now, when you run the script above, PHP will automatically read in the header and footer files, merge them with the HTML content, and display the complete page to you. Simple, isn't it?

Notice that you can even write PHP code inside the files being imported. When the file is first read in, the parser will look for <?php...?> tags, and automatically execute the code inside it. (If you're familiar with JavaScript, you can use this feature to replicate functionality similar to that of the onLoad() page event handler in JavaScript.)

PHP also offers the require_once() and include_once()functions, which ensure that a file which has already been read is not read again. This can come in handy if you have a situation in which you want to eliminate multiple reads of the same include file, either for performance reasons or to avoid corruption of the variable space.

A quick note on the difference between the include() and require()functions: the require()function returns a fatal error if the named file cannot be found and halts script processing, while the include() function returns a warning but allows script processing to continue.

Writing to Ma

After everything you've just read, you've probably realized that reading a file is not exactly brain surgery. So let's proceed to something slightly more difficult - writing to a file.

The steps involved in writing data to a file are almost identical to those involved in reading it: open the file and obtain a file handle, use the file handle to write data to it, and close the file. There are two differences: first, you must fopen() the file in write mode ('w' for write), and second, instead of using the fread() function to read from the file handle, use the fwrite() function to write to it. Take a look:

<?php

// set file to write
$file = '/tmp/dump.txt';
// open file
$fh = fopen($file, 'w') or die('Could not open file!');
// write to file
fwrite($fh, "Look, Ma, I wrote a file!\n") or die('Could not write to file');
// close file
fclose($fh);

?>

When you run this script, it should create a file named dump.txt in /tmp, and write a line of text to it, with a carriage return at the end. Notice that double quotes are needed to convert \n into a carriage return.

The fopen(), fwrite() and fread() functions are all binary-safe, which means you can use them on binary files without worrying about damage to the file contents. Read more about many of the issues related to binary-safe file manipulation on different platforms at http://www.php.net/manual/en/function.fopen.php.

If I've spoiled you by showing you the one-line shortcut functions for file reads, let me damage you further by introducing you to the file_put_contents() function, new in PHP 5.0, which takes a string and writes it to a file in a single line of code.

<?php

// set file to write
$filename = '/tmp/dump.txt';
// write to file
file_put_contents($filename, "Look, Ma, I wrote a file!\n") or die('Could not write to file');

?>

Bear in mind that the directory in which you're trying to create the file must exist before you can write to it. Forgetting this important step is a common cause of script errors.

Information is Power

PHP also comes with a bunch of functions that allow you to test the status of a file - for example to find out whether it exists, whether it's empty, whether it's readable or writable, and whether it's a binary or text file. Of these, the most commonly used operator is the file_exists() function, which is used to test for the existence of a specific file.

Here's an example which asks the user to enter the path to a file in a Web form, and then returns a message displaying whether or not the file exists:

<html>
<head>
</head>
<body>

<?php
// if form has not yet been submitted
// display input box
if (!isset($_POST['file'])) {
?>

    <form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="post">
    Enter file path <input type="text" name="file">
    </form>

<?php
}
// else process form input
else {
// check if file exists
    // display appropriate message
if (file_exists($_POST['file'])) {
        echo 'File exists!';
        }
    else {
        echo 'File does not exist!';
    }
}
?>

</body>
</html>

There are many more such functions. Here's a brief list, followed by an example that builds on the previous one to provide more information on the file specified by the user.

· is_dir() - returns a Boolean indicating whether the specified path is a directory

· is_file() - returns a Boolean indicating whether the specified file is a regular file

· is_link() - returns a Boolean indicating whether the specified file is a symbolic link

· is_executable() - returns a Boolean indicating whether the specified file is executable

· is_readable()- returns a Boolean indicating whether the specified file is readable

· is_writable()- returns a Boolean indicating whether the specified file is writable

· filesize() - gets size of file

· filemtime() - gets last modification time of file

· filemtime() - gets last access time of file

· fileowner() - gets file owner

· filegroup() - gets file group

· fileperms() - gets file permissions

· filetype() - gets file type

This script asks for a file name as input and uses the functions above to return information on it.

<html>
<head>
</head>
<body>

<?php
/* if form has not yet been submitted, display input box */
if (!isset($_POST['file'])) {
?>

    <form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="post">
    Enter file path <input type="text" name="file">
    </form>

<?php
}
// else process form input
else {
    echo 'File name: <b>'.$_POST['file']  .'</b><br />';
/* check if file exists and display appropriate message */
if (file_exists($_POST['file'])) {
// print file size
echo 'File size: '.filesize($_POST['file']).' bytes<br />';
// print file owner
echo 'File owner: '.fileowner($_POST['file']).'<br />';
// print file group
echo 'File group: '.filegroup($_POST['file']).'<br />';
// print file permissions
echo 'File permissions: '.fileperms($_POST['file']).'<br />';
// print file type
echo 'File type: '.filetype($_POST['file']).'<br />';
// print file last access time
echo 'File last accessed on: '.date('Y-m-d', fileatime($_POST['file'])).'<br />';
// print file last modification time
echo 'File last modified on: '.date('Y-m-d', filemtime($_POST['file'])).'<br />';
// is it a directory?
if (is_dir($_POST['file'])) {
            echo 'File is a directory <br />';
        }
// is it a file?
if (is_file($_POST['file'])) {
            echo 'File is a regular file <br />';
        }
// is it a link?
if (is_link($_POST['file'])) {
            echo 'File is a symbolic link <br />';
        }
// is it executable?
if (is_executable($_POST['file'])) {
            echo 'File is executable <br />';
        }
// is it readable?
if (is_readable($_POST['file'])) {
            echo 'File is readable <br />';
        }
// is it writable?
if (is_writable($_POST['file'])) {
            echo 'File is writable <br />';
        }
    }
    else {
        echo 'File does not exist! <br />';
    }
}
?>

</body>
</html>

And here's what the output might look like:

File name: /usr/local/apache/logs/error_log
File size: 53898 bytes
File owner: 0
File group: 0
File permissions: 33188
File type: file
File last accessed on: 2004-05-26
File last modified on: 2004-06-20
File is a regular file
File is readable

Breaking Eggs

So now you know how to read a file, write to it, and test its status. Let's look at some examples of what you can do with this new-found power.

Let's go back to my Spanish omelette recipe. Let's suppose I'm feeling generous, and I decide that I'd like to hear what people really think about my culinary skills. Since I have a bunch of recipes that I'd like to share with people, and since they all look something like this:

SPANISH OMELETTE
INGREDIENTS:
- 1 chopped onion
- 1 chopped tomato
- 1/2 chopped green pepper
- 4 beaten eggs
- Salt and pepper to taste
METHOD:
1. Fry onions in a pan
2. Pour beaten eggs over onions and fry gently
3. Add tomatoes, green pepper, salt and pepper to taste
4. Serve with toast or bread

I need a quick way to convert them all into HTML so that they look presentable on my Web site. We've already established that I'm lazy, so fuggedaboutme re-creating the recipes in HTML. Instead, I'll have PHP do the heavy lifting for me:

<html>
<head></head>
<body>

<?php
// read recipe file into array
$data = file('/usr/local/stuff/that/should/be/elsewhere/omelette.txt');
/* first line contains title: read it into variable */
$title = $data[0];
// remove first line from array
array_shift($data);
?>

<h2><?php echo $title; ?></h2>

<?php
/* iterate over content and print it */
foreach ($data as $line) {
    echo nl2br($line);
}
?>

</body>
</html>

I've used the file() function to read the recipe into an array, and assign the first line (the title) to a variable. That title is then printed at the top of the page. Since the rest of the data is fairly presentable as is, I can simply print the lines to the screen one after the other. Line breaks are automatically handled for me by the extremely cool nl2br() function, which converts regular text linebreaks into the HTML equivalent, the <br /> tag. The end result: an HTML-ized version of my recipe that the world can marvel at. Take a look:

<html>
<head></head><body>
<h2>SPANISH OMELETTE
</h2>
INGREDIENTS:<br />
- 1 chopped onion<br />
- 1 chopped tomato<br />
- 1/2 chopped green pepper<br />
- 4 beaten eggs<br />
- Salt and pepper to taste<br />
METHOD:<br />
1. Fry onions in a pan<br />
2. Pour beaten eggs over onions and fry gently<br />
3. Add tomatoes, green pepper, salt and pepper to taste<br />
4. Serve with toast or bread<br />
</body>
</html>

If the elegance and creative simplicity of my Spanish omelette recipe has left you speechless, I'm not surprised - many people feel that way. Until you get your voice back: Ciao... and make sure you come back to work through Part Six of PHP 101, which discusses creating your own reusable functions.

A Little Knowledge

If you've been taking your regular dose of PHP 101, you know now enough about PHP to write simple programs of your own. However, these programs will be "procedural" or linear - the statements in them will be executed sequentially, one after another - simply because that's the only programming style I've used so far.

You know what they say about a little knowledge being a dangerous thing... as your PHP scripts become more and more complex, it's only a matter of time before you bump your head against the constraints of the procedural method, and begin looking for a more efficient way of structuring your PHP programs.

That's where Part Six of PHP 101 comes in. In this tutorial I'm going to introduce you to a new way of doing things, where code doesn't run in a straight line, but twists, leaps and bounds across the landscape of your script. Most of this activity is accomplished through a programming construct called a "function", and this tutorial teaches you how to build them (once), use them (many times), pass them arguments and have them return values, and generally make your scripts more compact, efficient and maintainable.

In Plain English

Ask a geek to define the term "function", and he'll probably mumble something about a function being "a block of statements that can be grouped together as a named entity." Since this is a tutorial on PHP, not an introductory course in Greek, I'll translate that for you: a function is simply a set of program statements which perform a specific task, and which can be "called", or executed, from anywhere in your program.

Every programming language comes with its own built-in functions, and typically also allows developers to define their own functions. For example, if I had a profit statement for the year on my desk, and I wanted to inflate each number by 35%, I could call my neighborhood accounting firm and get them to do it for me... or I could write a simple PHP function called cheatTheShareholders() and have it do the work for me (it's faster, plus PHP doesn't bill by the hour).

There are three important reasons why functions are a Good Thing™. First: user-defined functions allow you to separate your code into easily identifiable subsections - which are easier to understand and debug. Second: functions make your program modular, allowing you to write a piece of code once and then re-use it multiple times within the same program. And third: functions simplify code updates or changes, because the change needs only to be implemented in a single place (the function definition). Functions thus save time, money and electrons... and I know the electrons at least will thank you!

Monday Morning Blues

To see how a function works, look at the following example:

<?php

// define a function
function myStandardResponse() {
    echo "Get lost, jerk!<br /><br />";
}

// on the bus
echo "Hey lady, can you spare a dime? <br />";
myStandardResponse();

// at the office
echo "Can you handle Joe's workload, in addition to your own, while he's in Tahiti for a month? You'll probably need to come in early and work till midnight, but we are confident you can handle it. Oh, and we can't pay you extra because of budgetary constraints...<br />";
myStandardResponse();

// at the party
echo "Hi, haven't I seen you somewhere before?<br />";
myStandardResponse();

?>

Here's what the output might look like:

Hey lady, can you spare a dime?
Get lost, jerk!
 
Can you handle Joe's workload, in addition to your own, while he's in Tahiti for a month?
You'll probably need to come in early and work till midnight, but we are confident you can
handle it. Oh, and we can't pay you extra because of budgetary constraints...
Get lost, jerk!
 
Hi, haven't I seen you somewhere before?
Get lost, jerk!

(Sure it's rude, but it does demonstrate how a function allows you to reuse pieces of code.)

The first thing I've done in the script above is define a new function, with the function keyword. This keyword is followed by the name of the function, which in this case is myStandardResponse(). All the program code attached to that function is then placed within a pair of curly braces - and this program code can contain loops, conditional statements, calls to other user-defined functions, or calls to other PHP functions.

Of course, defining a function is only half of the puzzle; for it to be of any use at all, you need to "invoke" it. In PHP, as in a million other languages, this is accomplished by calling the function by its name, as I've done in the example above. Calling a user-defined function is identical to calling a built-in PHP function like echo() or explode().

Here's the typical format for a function:

function function_name (optional function arguments) {
    statement 1...
    statement 2...
    .
    .
    .
    statement n...
}

Having an Argument... or Two

Functions like the one you saw in the previous section print the same value every time you invoke them. While this is interesting the first six times, it can get boring on the seventh. What we need to do, to make these boring, unintelligent functions a little more exciting, is get them to return a different value each time they are invoked.

Enter arguments.

Arguments work by using a placeholder to represent a certain variable within a function. Values for this variable are provided to the function at run-time from the main program. Since the input to the function will differ at each invocation, so will the output.

To see how this works, look at the following function, which accepts a single argument and then prints it back after a calculation:

<?php

// define a function
function getCircumference($radius) {
    echo "Circumference of a circle with radius $radius is ".sprintf("%4.2f", (2 * $radius * pi()))."<br />";
}

// call a function with an argument
getCircumference(10);

// call the same function with another argument
getCircumference(20);

?>

In this example, when the getCircumference() function is called with an argument, the argument is assigned to the placeholder variable $radius within the function, and then acted upon by the code within the function definition.

It's also possible to pass more than one argument to a function. This is done using a comma-separated list, as the following example shows:

<?php

// define a function
function changeCase($str, $flag) {
/* check the flag variable and branch the code */
switch($flag) {
        case 'U':
            print strtoupper($str)."<br />";
            break;
        case 'L':
            print strtolower($str)."<br />";
            break;
        default:
            print $str."<br />";
            break;
    }
}

// call the function
changeCase("The cow jumped over the moon", "U");
changeCase("Hello Sam", "L");

?>

Here, depending on the value of the second argument, program flow within the function moves to the appropriate branch and manipulates the first argument.

Note that there is no requirement to specify the data type of the argument being passed to a function. Since PHP is a dynamically-typed language, it automatically identifies the variable type and acts on it appropriately.

Circles in the Sand

The functions on the previous page simply printed their output to the screen. But what if you want the function to do something else with the result? Well, in PHP, you can have a function return a value, such as the result of a calculation, to the statement that called it. This is done using a return statement within the function, as shown below:

<?php

// define a function
function getCircumference($radius) {
// return value
return (2 * $radius * pi());
}

/* call a function with an argument and store the result in a variable */
$result = getCircumference(10);

/* call the same function with another argument and print the return value */
print getCircumference(20);

?>

Here, the argument passed to the getCircumference() function is processed, and the result is returned to the main program, where it may be captured in a variable, printed, or dealt with in other ways.

You can even use the result of a function inside another function, as illustrated in this minor revision of the example above:

<?php

// define a function
function getCircumference($radius) {
// return value
return (2 * $radius * pi());
}

// print the return value after formatting it
print "The answer is ".sprintf("%4.2f", getCircumference(20));

?>

Return values need not be numbers or strings alone: a function can just as easily return an array (remember them?), as demonstrated in the following example:

<?php

/* define a function that can accept a list of email addresses */
function getUniqueDomains($list) {
/* iterate over the list, split addresses and add domain part to another array */
$domains = array();
    foreach ($list as $l) {
$arr = explode("@", $l);
$domains[] = trim($arr[1]);
    }
// remove duplicates and return
return array_unique($domains);
}

// read email addresses from a file into an array
$fileContents = file("data.txt");

/* pass the file contents to the function and retrieve the result array */
$returnArray = getUniqueDomains($fileContents);

// process the return array
foreach ($returnArray as $d) {
    print "$d, ";
}

?>

Assuming the file looked like this,

test@test.com
a@x.com
zooman@deeply.bored.org
b@x.com
guess.me@where.ami.net
testmore@test.com

the output of the script above would look like this:

test.com, x.com, deeply.bored.org, where.ami.net,

Note that the return statement terminates program execution inside a function.

Marching Order

The order in which arguments are passed to a function can be important. The following example requires that the name is passed as the first argument, and the place as the second.

<?php

// define a function
function introduce($name, $place) {
    print "Hello, I am $name from $place";
}

// call function
introduce("Moonface", "The Faraway Tree");

?>

This is the output:

Hello, I am Moonface from The Faraway Tree

In this example, if you reversed the order in which arguments were passed to the function, this is what you'd see:

Hello, I am The Faraway Tree from Moonface

And look what happens if you forget to pass a required argument altogether:

Warning: Missing argument 2 for introduce() in xx.php on line 3
Hello, I am Moonface from 

In order to avoid such errors, PHP allows you to specify default values for all the arguments in a user-defined function. These default values are used if the function invocation is missing some arguments. Here's an example:

<?php

// define a function
function introduce($name="John Doe", $place="London") {
    print "Hello, I am $name from $place";
}

// call function
introduce("Moonface");

?>

In this case the output would be:

Hello, I am Moonface from London

Notice that the function has been called with only a single argument, even though the function definition requires two. However, since default values are present for each argument in the function, the missing argument is replaced by the default value for that argument, and no error is generated.

The Amazing Shrinking Argument List

All the examples on the previous page have one thing in common: the number of arguments in the function definition is fixed. However, PHP 4.x also supports variable-length argument lists, by using the func_num_args() and func_get_args() commands. For want of a better name, these functions are called "function functions". Try wrapping your tongue around that while you look at the next example, which demonstrates how they can be used:

<?php

// define a function
function someFunc() {
// get the arguments
$args = func_get_args();

// print the arguments
print "You sent me the following arguments:";
    foreach ($args as $arg) {
        print " $arg ";
    }
    print "<br />";
}

// call a function with different arguments
someFunc("red", "green", "blue");
someFunc(1, "soap");

?>

Hmmm... if you're sneaky, you might have tried to pass someFunc() an array, and found that instead of displaying the elements of the array, it simply said "Array". You can fix this by adding a quick test for array arguments inside the function, as in this rewrite:

<?php

// define a function
function someFunc() {
// get the number of arguments passed
$numArgs = func_num_args();

// get the arguments
$args = func_get_args();

// print the arguments
print "You sent me the following arguments: ";
    for ($x = 0; $x < sizeof($args); $x++) {
        print "<br />Argument $x: ";
/* check if an array was passed and, if so, iterate and print contents */
if (is_array($args[$x])) {
            print " ARRAY ";
            foreach ($args[$x] as $index => $element) {
                print " $index => $element ";
            }
        }
        else {
            print " $args[$x] ";
        }
    }
}

// call a function with different arguments
someFunc("red", "green", "blue", array(4,5), "yellow");

?>

Going Global

Let's now talk a little bit about the variables used within a function, and their relationship with variables in the outside world. Usually, the variables used within a function are "local" - meaning that the values assigned to them, and the changes made to them, are restricted to the function space alone.

Consider this simple example:

<?php

// define a variable in the main program
$today = "Tuesday";

// define a function
function getDay() {
// define a variable inside the function
$today = "Saturday";
// print the variable
print "It is $today inside the function<br />";
}

// call the function
getDay();

// print the variable
print "It is $today outside the function";

?>

When you run this script, here is what you will see:

It is Saturday inside the function
It is Tuesday outside the function

In other words, the variable inside the function is insulated from the identically-named variable in the main program. Variables inside a function are thus aptly called "local" variables because they only exist within the function in which they are defined.

The reverse is also true: variables defined inside a function cannot be "seen" outside it. To illustrate, take a look at the next example and its output (or the lack of it):

<?php

// define a function
function getDay() {
// define a variable inside the function
$today = "Saturday";
}

getDay();
print "Today is $today";

?>

Here is the output:

Today is 

Depending on the error_reporting you have set up in php.ini, you might also see an error message:

Notice: Undefined variable: today in x1.php on line 10

However, I didn't say this can't be overcome. To have variables within a function accessible from outside it (and vice-versa), all you need to do is first declare them "global" with the - you guessed it! - global keyword.

Here is a rewrite of the earlier example, this time declaring the $today variable global:

<?php

// define a variable in the main program
$today = "Tuesday";

// define a function
function getDay() {
// make the variable global
global $today;

// define a variable inside the function
$today = "Saturday";
// print the variable
print "It is $today inside the function<br />";
}

// print the variable
print "It is $today before running the function<br />";

// call the function
getDay();

// print the variable
print "It is $today after running the function";

?>

And here is the output:

It is Tuesday before running the function
It is Saturday inside the function
It is Saturday after running the function

Thus, once a variable is declared global, it is available at the global level, and can be manipulated both inside and outside a function.

PHP also comes with so-called superglobal variables - variables that are always available, regardless of whether you're inside a function or outside it. You've already seen some of these special variables in action: the $_SERVER, $_POST and $_GET variables are all superglobals, which is why you can access things like the currently-executing script's name or form values even inside a function.

Superglobals are a Good Thing™, because they're always there when you need them, and you don't need to jump through any hoops to use the data stored inside them. Read more about superglobals and variable scope at http://www.php.net/manual/en/language.variables.predefined.php and http://www.php.net/manual/en/language.variables.scope.php.

Checking References

Any discussion about variables in and out of functions would be incomplete without some mention of the difference between "passing by reference" and "passing by value". So far, all the examples you've seen have involved passing arguments to a function "by value" - meaning that a copy of the variable was passed to the function, while the original variable remained untouched. However, PHP also allows you to pass "by reference" - meaning that instead of passing a value to a function, you pass a reference to the original variable, and have the function act on that instead of a copy.

Confusing? Well, this is probably easier to understand with an example. Let's start with this:

<?php

// create a variable
$today = "Saturday";

// function to print the value of the variable
function setDay($day) {
$day = "Tuesday";
    print "It is $day inside the function<br />";
}

// call function
setDay($today);

// print the value of the variable
print "It is $today outside the function";

?>

You've already seen this before, and you already know what the output is going to say:

It is Tuesday inside the function
It is Saturday outside the function

This is because when the getDay() function is invoked, it passes the value "Saturday" to the function ("passing by value"). The original variable remains untouched; only its content is sent to the function. The function then acts on the content, modifying and displaying it.

Now, look at how "passing by reference" works:

<?php

// create a variable
$today = "Saturday";

// function to print the value of the variable
function setDay(&$day) {
$day = "Tuesday";
    print "It is $day inside the function<br />";
}

// call function
setDay($today);

// print the value of the variable
print "It is $today outside the function";

?>

Notice the ampersand (&) before the argument in the function definition. This tells PHP to use the variable reference instead of the variable value. When such a reference is passed to a function, the code inside the function acts on the reference, and modifies the content of the original variable (which the reference is pointing to) rather than a copy. If you then try retrieving the value of the original variable outside the function, it returns the modified value:

.
It is Tuesday inside the function
It is Tuesday outside the function

Now you understand why I said no discussion about variables would be complete without mentioning the two ways of passing variables. This, of course, is what the global keyword does inside a function: use a reference to ensure that changes to the variable inside the function also reflect outside it. The PHP manual puts it best when it says "…when you declare a variable as global $var you are in fact creating a reference to a global variable". For more examples, read all about references at http://www.zend.com/manual/language.references.php.

And that just about concludes this tutorial. This time you've taken a big step towards better software design by learning how to abstract parts of your PHP code into reusable functions. You now know how to add flexibility to your functions by allowing them to accept different arguments, and how to obtain one (or more) return values from them. Finally, you've learned a little bit about how PHP treats variables inside and outside functions.

In Part Seven, I'll be showing you how to group related functions together into classes, and also telling you all about the cool new features in the PHP 5 object model. You definitely don't want to miss that one!

Patience Pays

Now that you've used PHP with MySQL and SQLite, you probably think you know everything you need to get started with PHP programming. In fact, you might even be thinking of cutting down your visits to Zend.com altogether, giving up this series for something flashier and cooler...

Uh-uh. Big mistake.

You see, while built-in database support makes programming with PHP easy, it isn't the only thing that makes PHP so popular. An easy-to-use XML API and new exception handling mechanism (in PHP 5), support for pluggable modules, and built-in session management are just some of the many other features that make PHP rock. And all these capabilities are going to be explored, in depth, right here in this very series, if you can just find it in yourself to hang around a little longer. So close your eyes, take a deep breath, and read on to find out all about this tutorial's topic: sessions and cookies.

Party Time

Maybe you heard this at the last party you went to: "HTTP is a stateless protocol, and the Internet is a stateless development environment".

No? Hmmm. Obviously, you don't go to the right parties.

In simple language, all this means is that HTTP, the HyperText Transfer Protocol that is the backbone of the Web, is unable to retain a memory of the identity of each client that connects to a Web site, and therefore treats each request for a Web page as a unique and independent connection, with no relationship whatsoever to the connections that preceded it. This "stateless environment" works great so long as you're aimlessly surfing the Web, but it can cause a serious headache for sites that actually depend on the data accumulated in previous requests. The most common example is that of an online shopping cart - in a stateless environment, it becomes difficult to keep track of all the items you've shortlisted for purchase as you jump from one catalog page to another.

Obviously, then, what is required is a method that makes it possible to "maintain state", allowing client connections to be tracked and connection-specific data to be maintained. And thus came about cookies, which allow Web sites to store client-specific information on the client system, and access the information whenever required. A cookie is simply a file, containing a series of variable-value pairs and linked to a domain. When a client requests a particular domain, the values in the cookie file are read and imported into the server environment, where a developer can read, modify and use them for different purposes. A cookie is a convenient way to carry forward data from one client visit to the next.

Another common approach is to use a session to store connection-specific data; this session data is preserved on the server for the duration of the visit, and is destroyed on its conclusion. Sessions work by associating every session with a session ID (a unique identifier for the session) that is automatically generated by PHP. This session ID is stored in two places: on the client using a temporary cookie, and on the server in a flat file or a database. By using the session ID to put a name to every request received, a developer can identify which client initiated which request, and track and maintain client-specific information in session variables (variable-value pairs which remain alive for the duration of the session and which can store textual or numeric information).

Sessions and cookies thus provide an elegant way to bypass the stateless nature of the HTTP protocol, and are used on many of today's largest sites to track and maintain information for personal and commercial transactions. Typically, you use a session to store values that are required over the course of a single visit, and a cookie to store more persistent data that is used over multiple visits.

PHP has included support for cookies since PHP 3.0, and built-in session management since PHP 4.0. Both these features are enabled by default, so you don't have to do anything special to activate them. Instead, scroll down and take a look at your first session.

The First Session

One of the standard examples used to demonstrate how a session works is the hit counter application. This is a simple counter that initializes a variable the first time you visit a Web page, and increments it each time you reload the page. The counter variable is stored in a session, which means that if you browse to another site and then return, the last saved value of the counter will be restored (so long as you didn't destroy the session by shutting down the browser in the interim).

Take a look at the code:

<?php

// initialize a session
session_start();

// increment a session counter
$_SESSION['counter']++;

// print value
echo "You have viewed this page " . $_SESSION['counter'] . " times";

?>

To see how this works, request the script above through your browser a few times. You will notice that the counter increases by 1 on each subsequent page load. If you open up two browser windows and request the same page in each one, PHP will maintain and increment individual session counters for each browser instance. The session ID is used to identify which client made which request, and recreate the prior saved environment for each individual session. This also means that if you visit one (or more) other Web sites during the same session and then return to the script above without shutting down your browser in the interim, your previous session will be retrieved and recreated for you.

Every session in PHP begins with a call to the session_start() function. This function checks to see whether a session already exists, and either restores it (if it does) or creates a new one (if it doesn't). Session variables can then be registered by adding keys and values to the special $_SESSION superglobal array, and can be accessed at any time during the session using standard array notation. In the example above, a key named counter has been added to the $_SESSION array. The first time a session is created, this key will have the value 0. On every subsequent request for the page during the same session, the previous value of the counter will be retrieved and incremented by 1.

If the example above doesn't work as advertised, check to make sure that the session.save_path variable in your php.ini file points to a valid temporary directory for your system. This value is hard-wired to /tmp by default, so if you're trying the example on a Windows system, you will need to edit it to C:\Windows\temp (or your system's temporary directory).

Remember Me

Here's another example, this one asking you to log in and then storing your login name and session start time as two session variables. This information is then used to display the total number of minutes the session has been active.

<?php

// initialize a session
session_start();
?>
<html>
<head></head>
<body>

<?php
if (!isset($_SESSION['name']) && !isset($_POST['name'])) {
// if no data, print the form
?>
    <form action="<?php echo $_SERVER['PHP_SELF']?>" method="post">
        <input type="text" name="name">
        <input type="submit" name="submit" value="Enter your name">
    </form>
<?php
}
else if (!isset($_SESSION['name']) && isset($_POST['name'])) {
// if a session does not exist but the form has been submitted
    // check to see if the form has all required values
    // create a new session
if (!empty($_POST['name'])) {
$_SESSION['name'] = $_POST['name'];
$_SESSION['start'] = time();
        echo "Welcome, " . $_POST['name'] . ". A new session has been activated for you. Click <a href=" . $_SERVER['PHP_SELF'] . ">here</a> to refresh the page.";
    }
    else {
        echo "ERROR: Please enter your name!";
    }
}
else if (isset($_SESSION['name'])) {
// if a previous session exists
    // calculate elapsed time since session start and now
echo "Welcome back, " . $_SESSION['name'] . ". This session was activated " . round((time() - $_SESSION['start']) / 60) . " minute(s) ago. Click <a href=" . $_SERVER['PHP_SELF'] . ">here</a> to refresh the page.";
}
?>
</body>
</html>

In this example, the presence or absence of a session variable is used to decide which of the three possible screens to display. The session start time is also recorded in $_SESSION['start'] with the time() function, which returns the total number of seconds between January 1 1970 and the current time. At a later stage, the value stored in $_SESSION['start'] is compared with the most current value of time() to calculate and display an (approximate) display of elapsed time.

It's important to note that the call to session_start() must appear first, before any output is generated by the script (assuming you're not using PHP's output buffering feature, which you can read about at http://www.php.net/manual/en/ref.outcontrol.php). This is because the PHP session handler internally uses in-memory cookies to store session data, and the cookie creation headers must be transmitted to the client browser before any output. If you ever see an error like this in one of your session-enabled pages:

Warning: Cannot send session cache limiter - headers already sent (output started at ...)

it's usually because somewhere, somehow, some output has found its way to the browser before session_start() was called. Even a carriage return or a blank space outside the PHP tags surrounding session_start() can cause this error, so watch out for them.

As noted previously, every session has a unique session ID, which PHP uses to keep track of different clients. This session ID is a long alphanumeric string, which is automatically passed by PHP from page to page so that the continuity of the session is maintained. To see what it looks like, use the session_id() function, as in this simple example:

<?php

// initialize a session
session_start();

// print session ID
echo "I'm tracking you with session ID " . session_id();

?>

When the user shuts down the client browser and destroys the session, the $_SESSION array will be flushed of all session variables. You can also explicitly destroy a session - for example, when a user logs out - by calling the session_destroy() function, as in the following example:

<?php

// initialize a session
session_start();

// then destroy it
session_destroy();

?>

In case you were wondering if you read that right - yes, before you can call session_destroy() to destroy a session, you must first call session_start() to recreate it.

Remember that $_SESSION is a superglobal, so you can use it inside and outside functions without needing to declare it as global first. The following simple example illustrates this:

<?php

// initialize a session
session_start();

// this function checks the value of a session variable
// and returns true or false
function isAdmin() {
    if ($_SESSION['name'] == 'admin') {
        return true;
    }
    else {
        return false;
    }
}

// set a value for $_SESSION['name']
$_SESSION['name'] = "guessme";
// call a function which uses a session variable
// returns false here
echo isAdmin()."<br />";

// set a new value for $_SESSION['name']
$_SESSION['name'] = "admin";
// call a function which uses a session variable
// returns true here
echo isAdmin()."<br />";

?>

You can read more about sessions and session handling functions at http://www.php.net/manual/en/ref.session.php.

Rules Of The Game

A session works by using an in-memory cookie, which explains why it's only active while the browser instance that created it is active; once the browser instance is terminated, the memory allocated to that instance is flushed and returned to the system, destroying the session cookie in the process. If you want longer-lasting cookies, you can use PHP's built-in cookie functions to write data to the user's disk as a cookie file, and read this data back as and when needed.

Before you start using cookies, there are a few things you should be aware of:

1. Since cookies are used to record information about your activities on a particular domain, they can only be read by the domain that created them

2. A single domain cannot set more than twenty cookies, and each cookie is limited to a maximum size of 4 KB

3. A cookie usually possesses six attributes, of which only the first is mandatory. Here they are:

o name: the name of the cookie

o value: the value of the cookie

o expires: the date and time at which the cookie expires

o path: the top-level directory on the domain from which cookie data can be accessed

o domain: the domain for which the cookie is valid

o secure: a Boolean flag indicating whether the cookie should be transmitted only over a secure HTTP connection

More information on cookies can be obtained from Netscape, the people who originally invented them. Visit http://www.netscape.com/newsref/std/cookie_spec.html for the Netscape cookie specification.

It's important to remember that, since cookies are stored on the user's hard drive, you as the developer have very little control over them. If a user decides to turn off cookie support in his or her browser, your cookies will simply not be saved. Therefore, avoid writing code that depends heavily on cookies; and have a backup plan ready in case cookie data cannot be retrieved from the client.

With that caveat out of the way, let's look at some simple cookie-handling code in PHP.

Meeting Old Friends

PHP offers a single function for cookie manipulation: setcookie(). This function allows you to read and write cookie files, as demonstrated in the following example:

<?php

if (!isset($_COOKIE['visited'])) {
// if a cookie does not exist
    // set it
setcookie("visited", "1", mktime()+86400, "/") or die("Could not set cookie");
    echo "This is your first visit here today.";
}
else {
// if a cookie already exists
echo "Nice to see you again, old friend!";
}

?>

To see how this works, request the page above through your browser a couple of times. The first time around, because no cookie has yet been set, the first message will be displayed. On all subsequent attempts, because the cookie has already been set, the client will be recognized and the second message will be displayed. Note that this works even if you terminate the browser instance, restart it and visit the page again - a marked difference from what happened in the session examples you saw earlier.

The setcookie() function accepts six arguments: the name of the cookie, its value, its expiry date, the domain, the path for which it is valid, and a Boolean value indicating its security state. As noted previously, only the name and value are mandatory, although the example above specifies both a top-level directory and an expiry date for the cookie (1 day) with the mktime() function, which works like the time() function described previously.

Cookie values are automatically sent to PHP from the client, and converted to key-value pairs in the $_COOKIE variable, a superglobal array similar to $_SESSION. Values can then be retrieved using standard associative array notation, as in the example above. Note that, as with sessions, calls to setcookie() must take place before any output is generated by the script, or else you'll see an error like this:

Warning: Cannot add header information - headers already sent by (output started at ... )

Form And Function

Here's another, slightly more complex example:

<?php

if (!isset($_POST['email'])) {
// if form has not been submitted
    // display form
    // if cookie already exists, pre-fill form field with cookie value
?>
    <html>
    <head></head>
    <body>

    <form action="<?php echo $_SERVER['PHP_SELF']?>" method="post">
        Enter your email address: <input type="text" name="email" value="<?php echo $_COOKIE['email']; ?>">
        <input type="submit" name="submit">
<?php
// also calculate the time since the last submission
if ($_COOKIE['lastsave']) {
$days = round((time() - $_COOKIE['lastsave']) / 86400);
            echo "<br /> $days day(s) since last submission";
        }
?>
    </form>

    </body>
    </html>
<?php
}
else {
// if form has been submitted
    // set cookies with form value and timestamp
    // both cookies expire after 30 days
if (!empty($_POST['email'])) {
setcookie("email", $_POST['email'], mktime()+(86400*30), "/");
setcookie("lastsave", time(), mktime()+(86400*30), "/");
        echo "Your email address has been recorded.";
    }
    else {
        echo "ERROR: Please enter your email address!";
    }
}
?>
</body>
</html>

In this case, the value entered into the form is stored as a cookie called email, and automatically retrieved to pre-fill the form field on all subsequent requests. This technique is frequently used by Web sites that require the user to enter a login name and password; by automatically pre-filling the username field in the login box with the value used in the last successful attempt, they save the user a few keystrokes.

This example also demonstrates how you can set more than one cookie for a domain, by calling setcookie() multiple times. In the example above, the time at which the data was entered is stored as a second cookie, and used to calculate the time elapsed between successive entries.

To remove a cookie from the client, simply call setcookie() with the same syntax you used to originally set the cookie, but an expiry date in the past. This will cause the cookie to be removed from the client system. Here's an example:

<?php

// delete cookie
setcookie("lastsave", NULL, mktime() - 3600, "/");

?>

Read more about cookies and the setcookie() function at http://www.php.net/manual/en/features.cookies.php and http://www.php.net/manual/en/function.setcookie.php.

Access Granted

As I said at the beginning of this tutorial, cookies and sessions are two different ways of making data "persistent" on the client. A session retains data for the duration of the session, while a cookie retains values for as long as you need it to. With that in mind, let's now look at an example that uses them both.

The application here is a simple user authentication system, where certain pages can only be viewed by users who successfully log in to the system. Users who have not been authenticated with a valid password are denied access to these "special" pages. The list of valid usernames and passwords is stored in a MySQL database, and PHP is used to verify a user's credentials and decide whether or not to grant access.

Assuming the MySQL database table looks like this

+-------+-----------------------------------------------+
| name | pass |
+-------+-----------------------------------------------+
| sue | 9565d44fd0fe4db59f073eea1db70f3ea258e10b |
| harry | 6e74234b8b552685113b53c7bff0f386c8cef8cf |
| louis | 6817dda51b64b8190029490d2811a4d9cb9cd432 |
| sam | bd17f8243e771a57cfbb06aa9a82bbf09fd2d90b |
| james | 792ec9b44d432c947ac6775b2b52326e9d08512f |
+-------+-----------------------------------------------+

with a unique username field and a password field created with the SHA1() function, here's the PHP script that does all the hard work:

<?php

if (isset($_POST['name']) || isset($_POST['pass'])) {
// form submitted
    // check for required values
if (empty($_POST['name'])) {
        die ("ERROR: Please enter username!");
    }
    if (empty($_POST['pass'])) {
        die ("ERROR: Please enter password!");
    }

// set server access variables
$host = "localhost";
$user = "test";
$pass = "test";
$db = "db2";

// open connection
$connection = mysql_connect($host, $user, $pass) or die ("Unable to connect!");

// select database
mysql_select_db($db) or die ("Unable to select database!");

// create query
$query = "SELECT * FROM users WHERE name = '" . $_POST['name'] . "' AND pass = SHA1('" . $_POST['pass'] . "')";

// execute query
$result = mysql_query($query) or die ("Error in query: $query. " . mysql_error());

// see if any rows were returned
if (mysql_num_rows($result) == 1) {
// if a row was returned
        // authentication was successful
        // create session and set cookie with username
session_start();
$_SESSION['auth'] = 1;
setcookie("username", $_POST['name'], time()+(84600*30));
        echo "Access granted!";
    }
    else {
// no result
        // authentication failed
echo "ERROR: Incorrect username or password!";
    }

// free result set memory
mysql_free_result($result);

// close connection
mysql_close($connection);
}
else {
// no submission
    // display login form
?>
    <html>
    <head></head>
    <body>
    <center>
    <form method="post" action="<?php echo $_SERVER['PHP_SELF']; ?>">
    Username <input type="text" name="name" value="<?php echo $_COOKIE['username']; ?>">
    <p />
    Password <input type="password" name="pass">
    <p />
    <input type="submit" name="submit" value="Log In">
    </center>
    </body>
    </html>
<?php
}

?>

Here, the values entered into the login box are integrated into a MySQL SELECT query, which is executed on the user table. If both username and password match, a single record will be returned, indicating that authentication succeeded; if they don't, no records will be returned, indicating that authentication failed.

Assuming authentication succeeds, a session is initialized, the $_SESSION['auth'] key is created and assigned a value of Boolean true, and the username is stored in a cookie for next time. The cookie will remain valid for 30 days, and will be used to pre-fill the username field in the login box on the next login attempt.

Of course, this isn't enough by itself. While the script above performs authentication and initializes both a session and a cookie if the user's credentials are validated, a security check must also be carried out on each of the restricted pages. Without this check, any user could bypass the login screen and simply type in the exact URL to each page to view it.

Since it is clear from the previous script that the session variable $_SESSION['auth'] can only exist if the user's credentials have been validated, it suffices to check for the presence of the $_SESSION['auth'] variable at the top of each restricted page, and grant access if that check returns true. Here's how:

<?php

// start session
session_start();
if (!$_SESSION['auth'] == 1) {
// check if authentication was performed
    // else die with error
die ("ERROR: Unauthorized access!");
}
else {
?>
    <html>
    <head></head>
    <body>
    This is a secure page. You can only see this if $_SESSION['auth'] = 1
    </body>
    </html>
<?php
}

?>

Pretty neat, huh? Only authenticated users will be able to see this page, because only their clients will have a session with the $_SESSION['auth'] variable in it. Everyone else will simply see an error message.

That's about it for this tutorial. In Part Eleven, I'll be telling you all about SimpleXML, the new XML processing toolkit that comes bundled with PHP 5. Make sure you come back for that!