PHP (now a recursive acronym for "PHP Hypertext Preprocessor", but originally "Personal Home Page Tools",) is a widely used open-source programming language used primarily for server-side applications, to develop dynamic web content. It can be seen as an open source alternative to Microsoft's Active Server Pages (ASP) system and to CGI/Perl system.
Its ease of use and similarity with the most common structured programming languages, most notably C and Perl, allows most experienced programmers to start developing complex applications with a minimal learning curve. It also enables experienced developers to get involved with dynamic web content applications without having to learn a whole new set of functions and practices.
One of the more attractive parts of PHP is that it is more than just a scripting language. Thanks to its modular design, PHP can also be used to develop GUI applications, and it can be used from the command line just as Perl or Python can be.
PHP allows, among other things, easy interaction with a large number of relational database systems (Oracle, DB2, MySQL, PostgreSQL, etc.), while maintaining a simple and straightforward syntax. PHP runs on every major operating system, including Unix, Linux, Windows, and Mac OS X and can interact with all major web servers. The official PHP website contains extensive online documentation. The Linux, Apache, MySQL, PHP (LAMP) architecture has become very popular in the industry as a way of cheaply deploying reliable, scalable, and secure web applications.
PHP was originally designed as a wrapper around Perl by Rasmus Lerdorf in 1994 to display his resume information and collect some data, such as how many hits it was generating. Others first used "Personal Home Page Tools" in 1995, which Lerdorf had combined with his own Form Interpreter to create PHP/FI. Zeev Suraski and Andi Gutmans, two Israeli developers of the Technion - Israel Institute of Technology rewrote the parser in 1997, forming the basis of PHP 3. They also changed the name to its current recursive form. After months in beta, the development team officially released PHP/FI 2 in November 1997. Public testing of PHP 3 began immediately and the official launch came in June 1998. Suraski and Gutmans started a new rewrite of PHP's core, producing the Zend engine in 1999. In May 2000, PHP 4 powered by the Zend Engine was released. Development continues toward PHP 5 with Zend Engine 2.78.
PHP is one of the most popular server-side scripting systems on the Web. It's been widely adopted since the release of version 4, which was the first version powered by the powerful Zend Engine from Zend Technologies.
According to Netcraft's April 2002 survey, PHP is now the most-deployed server-side scripting language, running on around 9 million of the 37 million domains in their survey. This is confirmed by PHP.net's own figures, which show PHP usage measured on a per-domain basis growing at around 5% per month. In May 2003, almost 13 million domains were using PHP, based on the same source. 
However, PHP is not the most commonly used tool if measurements are made on a per-page basis. Another estimate in March 2002, based on searching for Web pages by their suffix, places PHP in second place at 30% of measured pages, behind 48% using Microsoft's ASP, but also shows PHP growing rapidly in market share. However, this method is notoriously inaccurate for measuring PHP popularity as some PHP systems dispense with the file names, using only directories, while other sites tend to dispense with the .php extension.
Due to PHP's popularity, a new breed of programmers has emerged who are only familiar with PHP, which in turn forced open the door towards a command line interface for PHP, along with support for GUI functions, such as Gtk or ncurses support. This is a major step for PHP, because it represents its adoption as a genuine programming language (i.e. running autonomously on a stand-alone machine, as opposed to its original purpose of serving web pages to client machines from a server).
Parts or all of this document has been reprinted from http://www.wikipedia.org.