PHP代码函数检查信用卡号是否正确

使用以下PHP函数检查卡号是否是信用卡：

function validateCard ( $cardnumber ) 
{ 
   $cardnumber = preg_replace ( " /\D|\s/ " , "" , $cardnumber ) ; # strip any non-digits 
   $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 ;
}