PHP function to verify whether the credit card number is correct

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 ;
    }