Sokil\Mongo\Validator\CardNumberValidator::getMod PHP Method

    private function getMod($cardNumber)
    {
        $digitList = str_split($cardNumber);
        $digitListLength = count($digitList);
        for ($i = 0; $i < $digitListLength; $i = $i + 2) {
            $digit = $digitList[$i] * 2;
            if ($digit > 9) {
                $digit -= 9;
            }
            $digitList[$i] = $digit;
        }
        return array_sum($digitList) % 10;
    }