how does the PHP.net website work?

Hi, I am going to code a website with lots of page, and to give the same design to the whole site, I can't no more use only one page with specific include, for example index.php?page=test. I was wondering how the php.net was working, and even if I took several hours trying to understand your source code, I can't understand how it works. Could you please tell me in a few words how your layout and design engine is working to keep such simplicity? Thanks a lot, and sorry for my poor english. Olivier Brin.