<?php
function luhn(string $s)
{
	$l = strlen($s);
	$m = $l % 2 + 1;
	$r = 0;
	for ($i = 0; $i < $l; ++$i) {
		$t = $s[$i] * $m;
		if ($t > 9) /* if > 9, then we've got 1X, we're supposed to sum the digits, so second digit + 1, is equivalent to $t - 9 */
			$t -= 9;
		$r += $t;
		$m = 3 - $m;
	}
	return 10 - ($r % 10);
}

function luhn_check(string $s) : bool
{
	$l = strlen($s);
	$c = $s[$l-1];
	$r = luhn(substr($s, 0, $l - 1));
	return $r == $c;
}
?>