About "wrapping" a forum into your own design.

I am making a page with css for styling and php for browsing and dynamic content. I use a simple switch statement to get the main content of each page into the same <div> tag like this: <div id="main"><?php switch(@$_GET['maintext']) { default; echo "Welcome to the official website of bladablada! Feel free to navigate around and get to now us better"; $maintitle="WELCOME"; break; case "news": echo "A bunch of news"; $maintitle="NEWS"; break; case "about": include ("about.php"); $maintitle="ABOUT"; break; case "gallery": include ("gallery.php"); $maintitle="SCREENSHOTS"; break; case "stats": include ("stats.php"); $maintitle="STATISTICS"; break; case "forum": include ('forum/index.php'); $maintitle="FORUM"; break; } ?></div> However. I am not that competent in PHP to make my own forum, so I use a premade forum. Invision Power Board actually. What I want to do is to get all the links I press in the forum (after loading "forum/index.php" into the div-tag with id=main) to show up in the same area. Not sure if this is a php question actually, but... What happens now is that when I press the link "Forum" in my main navigation on my site, the index.php of course loads into the div-tag with id=main. BUT when I press a topic or something within this page (index.php), it opens up in a new page. I know why, but I wonder if there is a way to get the rest of the links in the forum to stay within my div-tag other than tracking down all the variables and such in the forumscripts and alter them? (That's a hell of alot java and php to work through) Any help is appreciated.