Ajax :: Redirect / Refresh And Variable Set / Check

This is my first adventure into PHP/Ajax/MySql/Javascript and is pretty much my first web project. I've been trying to tackle this problem for a little over a week now. My main objective is that I want to display php content pages inside a div on the index page without refreshing the page (iframe style). I don't want to stop search engines indexing the individual pages so I need a mechanism to redirect to the index page so that the page is displayed properly. I'm not bothered if it takes them to the exact page and I'm redirecting them to the section of the site where they the page is kept.

I also want a mechanism to redirect if a second search engine link is clicked that belongs to the individual page (ie image links that belong to the same page). This is only a problem because the php session has already been opened and a php variable has been set by the first redirect. All the redirections are taken care of by PHP and I got the redirection of multiple instances working ok by setting and unsetting session variables. This PHP code is included on every content page.

But, when I added links to the index page and try to navigate between pages, it all goes pear-shaped.When the links are clicked it calls up the Javascript code, inside which a PHP variable is set (I dunno if this is the "done" thing, but I came across the solution somewhere on the Interweb). This is the first thing checked when the content pages are called up. If the variable isn't set, then it runs through the redirect options.

The problem is that after clicking the link when the page displays it always refreshes the index page which I don't want it to do and which is why I'm thinking it's the setting of the PHP variable inside the Javascript that's going wrong. (Also why this question ended up in this section). Is there any way of getting this going without passing the variable into the url (not that I'm against this option)? Index page code, PHP Code (before HTML)...

Code:
<?php
if(!isset($_SESSION))
{
session_start();
}
// set the session var to the index page url which I think remains until browser close
$_SESSION['pagecheck'] = "http://".$_SERVER['HTTP_HOST'].$_SERVER['REQUEST_URI'];
// set the section var if it hasn't already been set by one of the content pages
if(isset($_SESSION['sectNum'])!= "undefined")
{
$_SESSION['sectNum'] = "0";
}
function getSection() .....

Additionally, are there any potential security threats that should be tackled (bearing in mind that I don't want to use cookies, I'm not against this either but decided if they are switched of it may become an issue, and I'm not going to be handling any sensitive information). I really don't want to use Iframes due to any javascript switching off related problems (yes there is a javascript testing function included in my full code!).