One definition: PHP competes with Visual Basic and C++ as the third most popular programming language behind Java and C, based on world wide availability of practitioners, courses and vendors. Available under the PHP License, PHP is an open source language and considered to be free software by the Free Software Foundation.
Another definition: The PHP Hypertext Preprocessor is a programming language that allows web developers to create dynamic content that interacts with databases. PHP is basically used for developing web based software applications.
And one more for good measure: PHP is a server-side, cross-platform, HTML embedded scripting language that lets you create dynamic web pages. PHP-enabled web pages are treated just like regular HTML pages and you can create and edit them the same way you normally create regular HTML pages.
For a great educational resource, read http://en.wikipedia.org/wiki/PHP