PHP is a recursive acronym for PHP: Hypertext Preprocessor. It is a popular server-side scripting language designed specifically for integration with HTML, and is used (often in conjunction with MySQL) in Content Management Systems and other web applications. It is available on many platforms, including Windows, Unix/Linux and Mac OS X, and is open source software.

WordPress is written using PHP and requires it for operation.

Reference http://codex.wordpress.org/Glossary