I had a similar problem when designing a website last year, I knew nothing about PHP, and wanted to do something very similar.
Provided you hvae PHP enabled, just slot something like this into the code.
Code:
<div id="header">
<?php include 'header.php'; ?>
</div>
Basically all the ?php...........? does is temporarily invoke the PHP interpreter.
And obviously in header.php, is your header.
Jobs a goodun. Obviously you have to put the above code into every page, but it's good for making changes, and not having to edit millions of pages.
I assume that's the sort of thing you're talking about?