Mine

$cache=true;
if ($cache) {
    $offset = 60*60*4 ;
    header("Cache-Control: max-age=".$offset);
    header("Pragma:Cache-Control: max-age=".$offset);
    $expire = "Expires: " . gmdate("D, d M Y H:i:s", time() + $offset) . " GMT";
    header($expire);
} else {
    header("Pragma: no-cache");
}


$url=$_GET['u'];
if (!preg_match('/server.djingle.fr/',$url)) {
    if ($url=='undefined') {
        readfile($skin_path.'/img/contenu.gif');
    }
    die();
}

if (preg_match('/gif$/i',$url)) {
    header('Content-Type: image/gif');
} elseif (preg_match('/jpe?g$/i',$url)) {
    header('Content-Type: image/jpg');
} elseif (preg_match('/png$/i',$url)) {
    header('Content-Type: image/png');
}
readfile($url);
?>