Ezhik,
Итого (+/-)
$users = mysql_result(mysql_query("SELECT COUNT(*) FROM `users` WHERE `lastdate` > '" . (time() - 300) . "'"), 0);
$guests = mysql_result(mysql_query("SELECT COUNT(*) FROM `cms_sessions` WHERE `lastdate` > '" . (time() - 300) . "'"), 0);
$totall = $users + $guests;
echo '<div class="menu">' . $totall . '</div>';