Constant time string comparison function in PHP.

const_time_strcmp.php

function const_time_strcmp($a, $b) {
	// inputs must be strings
	if(!is_string($a) || !is_string($b))
		return false;
	// strings must be the same length
	if(strlen($a) !== strlen($b))
		return false;
	// result will be all zero if values are equal
	$c = $a ^ $b;
	// add up the byte values. if result is zero then strings are identical
	return array_sum(array_map('ord', str_split($c))) === 0;
}