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;
    }
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;
    }
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 ; }
Return the checksum char of luhn algorithm

@param string $string
@return string
Return the checksum char of luhn algorithm