PHP信用卡卡号验证函数

/**
     * 验证银行卡号是否是信用卡
     * @param $cardnumber
     * @return bool
     */
    function validateCard ($cardnumber){
        $cardnumber = preg_replace ("/\D|\s/","", $cardnumber);
        $cardlength = strlen($cardnumber);if($cardlength !=0){
            $parity = $cardlength %2;
            $sum =0;
            for($i=0; $i<$cardlength; $i++){
                $digit = $cardnumber[$i];
                if($i %2== $parity) $digit = $digit *2;
                if($digit >9) $digit = $digit -9;
                $sum = $sum + $digit;
            }
            $valid =($sum %10==0);
            return $valid;
        }
        return false;
    }