See if this works for you, you need prototype.
Demo: http://calebmingle.com/subnav/
index.html:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>Subnavigation Test</title>
<script type="text/javascript" src="js/prototype.js"></script>
<script type="text/javascript" src="js/scriptaculous.js"></script>
<script type="text/javascript">
function change_nav(nav) {
new Ajax.Updater('subnav', 'subnav.php?nav=' + nav);
}
change_nav('home');
</script>
</head>
<body>
<a href="#" onclick="javascript:change_nav('home');">Home</a> || <a href="#" onclick="javascript:change_nav('us');">About Us</a> || <a href="#" onclick="javascript:change_nav('habbo');">Habbo</a> <br /><hr /><br />
<div id="subnav"></div>
</body>
</html>
subnav.php
<?php
//
// subnav.php
// Subnavigation Script
//
// Created by Caleb Mingle on 2009-08-12.
// Copyright 2009 MacDowell Diesel Performance. All rights reserved.
//
// ================================================== =========
// = Switches the navigation based on what the user provides =
// ================================================== =========
switch($_GET["nav"]) {
case "home":
echo '<a href="home.php">Home</a> | <a href="home2.php">Other Home</a>';
break;
case "us":
echo '<a href="about.php">About Us</a> | <a href="contact.php">Contact Us</a>';
break;
case "habbo":
echo '<a href="guides.php">Guides</a> | <a href="badges.php">Badges</a>';
break;
}