php信誉卡卡号验证函数

介绍一个php信用卡卡号验证函数，可以验证一个卡号是否是信用卡。

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;}