Steven,
Here is something from my limited knowledge of PHP.
If you have any code that is repeated over more than one page, copy that code and save it as a php file. As an example, if you have the top of every page on your site with the same logo, address, phone number, etc. you can remove all that code and put it in a single file. I would call it masthead.php or something like that. I would put that file in a folder off my root folder and name that folder "includes" so I know where it is.
Now every page that has that masthead, will have this single line of code:
<?php include($_SERVER['DOCUMENT_ROOT']."/includes/masthead.php") ?>
If you ever need to edit the masthead, it is much easier to edit a single file instead of trying to edit every page and hope you don't miss any pages.