PHP验证信用卡卡号函数

/**
 * 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;
}