Here is a very rudimentary page presenting the four major Brahmi descended scripts of South East Asia. The Indic transcription is given in the first column to indicate the Brahmi character from which the letters are descended. When no Brahmi letter is given, then the letter was devised for that alphabet from another character. Since these language just chose letters that suited their languages, you'll find missing and added letters. Simple transcription for each language is provided. The reason an alphabet may have, for example, six letters "pho", is because of tone. Tone is not dealt with these charts, but each letter has an inherent tone which is modified through other letters, marks, and vowel markers. Enjoy!
Aspirant & Misc