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;
}