[PHP] masking a credit card number

So if you have ever had to create a shopping cart and on the receipt page you want to display the last 4 digits of the credit card number and just x out the rest well you would prolly create a bit of code like on the php.net comments such as:

function mask ( $str, $start = 0, $length = null ) {
   $mask = preg_replace ( "/\S/", "*", $str );
   if ( is_null ( $length )) {
       $mask = substr ( $mask, $start );
       $str = substr_replace ( $str, $mask, $start );
   } else {
       $mask = substr ( $mask, $start, $length );
       $str = substr_replace ( $str, $mask, $start, $length );
   }
   return $str;
}

But of course I don't like that solutions so here is my version:

$maskedCcn = substr_replace(
   preg_replace( '/\S/', 'X',
       preg_replace( '/[^0-9]*/', null, $cardNumber );
   ),
   substr( $maskedCcn, -4 ),
   strlen( $maskedCcn ) - 4,
   4
);