There are two easy things you can do to that page using PHP:
Right now, you are calling separate HTML files inside of a frame in order to create sub-pages. Instead of that, you could just write up your page template as separate file and simply have PHP automatically stick that header and footer into every single one of your pages. Then each individual page's code will look like:
PHP Code:
include("header.php");
<p>Oh man, what a cool page I have!</p>
include("footer.php");
Header.php and footer.php will contain all the boring HTML markup that is the same on every page, freeing up each file to just be the relevant code for that area. You can google around a bit to find some more information on these techniques, and I highly recommend taking advantage of the php.net website, which lets you search for information about almost anything you would ever want to know about PHP commands and syntax.
If you get comfortable with that, you can get really fancy and build a membership system.
This phpfreaks.com tutorial is what taught me how to create login systems that I have since used on a few of my own projects. It's extremely step by step, but by the end you should have a solid understanding of what each piece of the system is doing when a user logs in.
You could use a user login system to protect sensitive documents, set up some sort of communication system, or
just to be cool-looking.
Good luck! PHP is some pretty amazing stuff.