i got this one tested:

<?php # $Id: exit.php,v 1.5.1 2004/07/06 00:39:00 leeps Exp $
include_once 'serendipity_config.inc.php';

$url = $serendipity['baseURL'];

if (isset($_GET['url']) && !empty($_GET['url'])) {

$found_id = false;

$url = str_replace('&', '&', base64_decode($_GET['url']));
if ...