What is PHP?

The endless possibilities of the PHP scripting language and a great community of users has made it one of the most popular open-source languages. For all you people living outside the UNIX world, Open Source means it doesn’t cost anything. You can use it as much as you want and where you want, and nobody will ever charge you thousands of dollars for licenses and support. Even though it was originally conceived as a set of macros to help coders maintain personal home pages, its name grew a lot more from its purpose. Since then, PHP’s capabilities have been extended, taking it beyond a set of utilities to a full-featured programming language, capable of managing huge database-driven online environments.

PHP: Hypertext Preprocessor.
It is programming language used in dyanmic web pages. It used used with MySQL in the foundation for many online Bulletin Boards.
PhpBB and PunBB are two PhP based forum boards that together with MySQL provide the framework for many online forums