A PHP class for checking numbers trough the popular Luhn algorithm. It's also able to get the digit which you'll have to add that then Lun-check gets true.

Luhn.php

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<?php
class Luhn {
    public static function getDigit($number) {
        $number .= 0;
        $result = self::calc($number);
        return (10 - $result);
    }

    public static function calc($number) {
        $sum = 0;
        $numDigits = strlen($number) - 1;
        $parity = $numDigits % 2;
        for ($i = $numDigits; $i >= 0; $i--) {
            $digit = substr($number, $i, 1);
            if (!$parity == ($i % 2)) {$digit <<= 1;}
            $digit = ($digit > 9) ? ($digit - 9) : $digit;
            $sum += $digit;
        }
        return ($sum % 10);
    }

    public static function check($number) {
        return (0 == self::calc($number));
    }
}
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