protected function getChecksum($string)
    {
        $checksum = '';
        foreach (str_split(strrev($string)) as $i => $d) {
            $checksum .= ($i % 2 === 0) ? ($d * 2) : $d;
        }
        return (10 - array_sum(str_split($checksum)) % 10) % 10;
    }