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;
 }