Luhn algorithm? Verify that the card is valid

Luhn algorithm

1. Starting with the last digit of the card number, the inverse adds the odd digits (1, 3, 5, and so on).

2, starting from the last digit of the card number, the inverse of the even digit number, first multiplied by 2 (if the product is two digits, then subtract 9), and then sum.

3, the sum of odd digits plus even digits sum, the result should be divisible by 10.

function Checkcard ($card) {    $num = 0;    $card = Str_split (Trim ($card));    Krsort ($card);    $i = 1;    foreach ($card as $val) {        if ($i% 2) {//odd            $num + = $val;        } else {//even            $n = $val * 2;            if ($n > 9) $n-= 9;            $num + = $n;        }        $i + +;    }    Return (($num% 10) = = 0);}

Luhn algorithm? Verify that the card is valid