BITXOR var1,var2
This statement do a bitwise XOR between var1 and var2; that is, every corresponding bit of the variables (from left to right) is logically added with an exclusive OR and result is overlayed in var1.
Logically XORed mean: 1 x 1 = 0; 1 x 0 = 0; 0 x 1 = 0; 0 x 0 = 0.
This bitwise XOR is ever done no matter of what type of variables are used and its length correspond to the shortest length between var1 and var2.