Background
PHP is a popular open-source programming language used primarily for developing server-side applications and dynamic web content, and more recently, other software. The name is a recursive acronym for “PHP: Hypertext Preprocessor”.
PHP is the result of the collective efforts of many contributors dating back to 1994, most notably Rasmus Lerdorf (PHP’s original developer), Zeev Suraski and Andi Gutmans
PHP is currently one of the most popular server-side scripting systems on the Web.
PHP allows interaction with a large number of relational database management systems, such as MySQL, Oracle, IBM DB2, Microsoft SQL Server, PostgreSQL and SQLite.
Due to its modular design, PHP is also used to develop GUI applications (using PHP-GTK), and can be used from the command line just like Perl or Python.
Source: http://en.wikipedia.org/wiki/Php
PHP is one of the best documented languages available for web development today. The official PHP website contains extensive documentation.