Such a vague question won't get a very detailed answer but
$last_5_urls = $_SESSION['last_5_urls'];
$last_5_urls[] = current url;
$last_5_urls = array_slice($last_5_urls, -5);
$_SESSION['last_5_urls'] = $last_5_urls;
keeps a rolling buffer of the last 5 urls in a session variable.