The PHP Hypertext Preprocessor is a programming language which processes dynamic content stored in databases. Many Content Management Systems are built upon the PHP language.
Any time you put information into your Content Management System, PHP inserts it into a database. Likewise, anytime you retrieve information to display on your web page, PHP is used. PHP can do many other tasks other than act as liason to a database
PHP runs best with a PHP accelerator. The PHP accelerator caches the php scripts, preventing regeneration of the same information over and over on the server.