PHP is a free, multi platform computer programming language. It is most used to develop dynamic web sites that are more than static HTML pages. The language is very easy to learn and use, and has an extensive list of add ons that allow it to do all kinds of stuff, from storing data in databases, pulling data from other web sites, sending emails, and more.

PHP is often used with MySQL, which is a free SQL Relational database.

This language is used to power Drupal, the Content Management System that is used to run this web site. 

Here are some resources on the web for learning PHP and getting started with it:

• Introductory PHP Tutorial.
• Database driven web sites using PHP and MySQL.
• A free PHP PDF book : PHP 5 Power Programming by Gutmans, Rethans, and Bakken.
• Another PHP and MySQL tutorial.
• The PHP reference manual with user comments.
• PHP Tutorial in 7 parts.
• PHP/MySQL Tutorial in 8 parts.
• A collection of PHP tutorials at PHP Freaks.