PHP originally stood for Personal Home Page. It began in 1994 as a set of Common Gateway Interface binaries written in the C programming language by the Danish/Greenlandic programmer Rasmus Lerdorf. Lerdorf initially created these Personal Home Page Tools to replace a small set of Perl scripts he had been using to maintain his personal homepage. He combined these binaries with his Form Interpreter to create PHP/FI, which had more functionality. PHP/FI included a larger implementation for the C programming language and could communicate with databases, enabling the building of simple, dynamic web applications. Lerdorf released PHP publicly on June 8, 1995 to accelerate bug location and improve the code. This release was named PHP version 2 and already had the basic functionality that PHP has today.

Originally designed to create dynamic web pages, PHP's principal focus is server-side scripting. PHP has also attracted the development of many frameworks that provide building blocks and a design structure to promote rapid application development (RAD).

The LAMP and WAMP architectures have become popular in the web industry as a way of deploying web applications. PHP is commonly used as the P in this bundle alongside Linux, Apache and MySQL, although the P may also refer to Python or Perl.

PHP is similar to other server-side scripting languages that provide dynamic content from a web server to a client. It is the most used open source web development language. It must be noted that the extent of its usage has generated the need for a vast mix of programming skills.

There have been 5 versions released so far. Version 6.0 will have Unicode capability.

• As of April 2007, over 20 million Internet domains were hosted on servers with PHP installed, and PHP was recorded as the most popular Apache module. Significant websites are written in PHP including the user-facing portion of Facebook and Wikipedia.
• In addition to server-side scripting, PHP can be used to create stand-alone, compiled applications and libraries, it can be used for shell scripting.

If it's related to PHP, you'll find it at php.net

• PHP and MySQL Web Development by Luke Welling and Laura Thomson
• Sam's Teach Yourself PHP by Matt Zandstra