A Recursive Acronym for PHP: Hypertext Preprocessor
A widely used Web ProgrammingLanguage that can be embedded into HTML. It's part of LAMP and now attempting to become a general purpose scripting language. PHP.net have a great introductory tutorial. The online manual is very useful.
This Wiki is written in PHP. Other cool things written in PHP include Cacti, most WebMail clients and The Eight Ball
PHP has an interface for installing new modules, much like Perl's CPAN archive, called PEAR.