An excellent tutorial that addresses your question can be found at Ibm's DeveloperWorks website at http://www.ibm.com/developerworks/edu/os-dw-os-phpajax-i.html
A short version of this tutorial is this:
1 - On a the PHP server, have a page 'panels-ajax.php' with the one line:
<?php
require('content/panel-'.$_GET['panel_id'].'.html');
?>
On the same server, have your pages-to-fill-in-the-div-tag in a folder named 'content'.
For now, just stick one page in there, named panel-01.html
(If you have 10 pages, name them panel-0.html to panel-9.html)
The main page goes like:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head>
<title>Create a Content Management System with PHP</title>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<style type="text/css">
<!-- Using span instead of regular anchors -->
span:visited{ text-decoration:none; color:#293d6b; }
span:hover{ text-decoration:underline; color:#293d6b; }
span {color:#293d6b; cursor: pointer}
</style>
<script type="text/javascript">
var request;
var dest;
function processStateChange()
{
if (request.readyState == 4)
{
contentDiv = document.getElementById(dest);
if (request.status == 200)
{
response = request.responseText;
contentDiv.innerHTML = response;
}
else
{
contentDiv.innerHTML = "Error: Status "+request.status;
}
}
}
function loadHTML(URL, destination)
{
dest = destination;
if (window.XMLHttpRequest)
{
request = new XMLHttpRequest();
request.onreadystatechange = processStateChange;
request.open("GET", URL, true);
request.send(null);
}
else if (window.ActiveXObject)
{
request = new ActiveXObject("Microsoft.XMLHTTP");
if (request)
{
request.onreadystatechange = processStateChange;
request.open("GET", URL, true);
request.send();
}
}
}
</script>
</head>
<body>
<!-- Load Menu buttons: (only one shown) -->
<span onclick="loadHTML('panels-ajax.php?panel_id=0', 'content')">Managing content</span>
<!-- Location where the pages will be shown -->
<div id="content"></div>
</body>
</html>