luhn.php

<?php
function luhnCheck($number) {
	$digits = str_split($number);
	for( $i = (count($digits) - 2); $i >= 0; $i -= 2 ) {
		$digits[$i] *= 2;
	}
	$digits = str_split(implode($digits));
	return (array_sum($digits) % 10) === 0;
}

// returns true if a number passes the luhn (mod 10) check.
// (http://en.wikipedia.org/wiki/Luhn_algorithm)