For secure and slow string comparisons. This helps against timing attacks on secret strings.

slow_compare.php

<?php
function slow_compare($str1, $str2, $length = null) {
	$out = ($str1_len = strlen($str1 .= chr(0))) - ($str2_len = strlen($str2 .= chr(0)));
	if (!is_int($length)) $length = $str1_len;
	for ($i = 0; $i < $length; $i++)
		$out |= ord($str1[$i % $str1_len]) ^ ord($str2[$i % $str2_len]);
	return 0 === $out;
}