<?php
function is_valid_nums($s){
$s = strrev(preg_replace('/[^\d]/','',$s));
$sum = 0;
for ($i = 0,$j=strlen($s); $i < $j; $i++) {
if ($i % 2 ==0) {
$val = $s[$i];
}else {
$val = $s[$i]*2;
if ($val >9) {
$val -=9;
}
$sum += $val;
}
return(($sum % 10) == 0);
}
if (! is_valid_nums($_POST['nums'])) {
echo 'the num is invalid.';
}
}
?>

Please be aware that this functions will only work if the passed argument is a string, else it will fail.
For more info read:

http://lamp-ios.com/?p=256

It has to do with 1 thing:
1. Your platform (32 or 64 bit)

Be aware of this

Here is a simple implementation:

/**
 * @see http://en.wikipedia.org/wiki/Luhn_algorithm
 */
private function checksum ($card_number) {
    $card_number_checksum = '';

    foreach (str_split(strrev((string) $card_number)) as $i => $d) {
        $card_number_checksum .= $i %2 !== 0 ? $d * 2 : $d;
    }

    return array_sum(str_split($card_number_checksum)) % 10 === 0;
}

You can simply typecast whatever input you have into a string before applying it to the function or you could incorporate a step to first typecast your arguments into strings.

This function works perfectly if your number is already in a 16-character string, is_valid_luhn("4111111111111111")

But the function will fail if the string contains any symbols or spaces, is_valid_luhn("4111 1111 1111 1111") = false

This can be easily fixed by placing a preg_replace at the beginning of the function to strip all non-digit characters.

$number = preg_replace("/[^0-9]/", "", $number);