Create the header and the footers for the information you want. On the top piece of the php, the variable of $page == what ever it's set through the get @!#$.
Next, a simple enough way of including whatever page you want is just doing a virtual include w/ the file name set in a php echo for $page. Not to tough. Check it out when the http address is this:
http://www.whatever.com/index.php?page=info
<?php
//Header stuffems
?>
<!--#include virtual="/path/<? echo ($page."html"); >?" -->
<?php
//Footer stuff
?>
The idea...It'll print out in the virtual include the contents of info.html. No need to add the .thml in the header, makes you seem all the more code savvy to your users ;-).