PHP is a reflective programming language originally designed for producing dynamic web pages. PHP is used mainly in server-side scripting, but can be used from a command line interface or in standalone graphical applications. Textual User Interfaces can also be created using ncurses.

The main implementation is produced by The PHP Group and released under the PHP License. It is considered to be free software by the Free Software Foundation. This implementation serves to define a de facto standard for PHP, as there is no formal specification.

Currently, two major versions of PHP are being actively developed: 5.x and 4.4.x; on July 13, 2007, the PHP group announced that active development on PHP4 will cease by December 31, 2007, however, critical security updates will be provided until August 8, 2008.

PHP was written as a set of CGI binaries in the C programming language by the Danish/Greenlandic programmer Rasmus Lerdorf in 1994, to replace a small set of Perl scripts he had been using to maintain his personal homepage.[4] Lerdorf initially created PHP to display his résumé and to collect certain data, such as how much traffic his page was receiving. Personal Home Page Tools was publicly released on 8 June 1995 after Lerdorf combined it with his own Form Interpreter to create PHP/FI (this release is considered PHP version 2).[5]

Zeev Suraski and Andi Gutmans, two Israeli developers at the Technion IIT, rewrote the parser in 1997 and formed the base of PHP 3, changing the language's name to the recursive initialism PHP: Hypertext Preprocessor. The development team officially released PHP/FI 2 in November 1997 after months of beta testing. Public testing of PHP 3 began and the official launch came in June 1998. Suraski and Gutmans then started a new rewrite of PHP's core, producing the Zend Engine in 1999.[6] They also founded Zend Technologies in Ramat Gan, Israel, which actively manages the development of PHP.

In May 2000, PHP 4, powered by the Zend Engine 1.0, was released. The most recent update released by The PHP Group, is for the older PHP version 4 code branch which, as of May 2007, is up to version 4.4.7. PHP 4 will be supported by security updates until 31 December 2007[7].

On July 13, 2004, PHP 5 was released powered by the new Zend Engine II. PHP 5 included new features such as:[8]

Improved support for object-oriented programming
The PHP Data Objects extension, which defines a lightweight and consistent interface for accessing databases
Performance enhancements
Better support for MySQL
Embedded support for SQLite
Integrated SOAP support
Data iterators
Error handling via exceptions
The latest stable version, PHP 5.2.3, was released on June 1, 2007.

PHP generally runs on a web server, taking PHP code as its input and creating Web pages as output, however it can also be used for command-line scripting and client-side GUI applications. PHP can be deployed on most web servers and on almost every operating system and platform free of charge. The PHP Group also provides the complete source code for users to build, customize and extend for their own use.

Server-side scripting
Originally designed to create dynamic web pages, PHP's principal focus is server-side scripting. While running the PHP parser with a web server and web browser, the PHP model can be compared to other server-side scripting languages such as Microsoft's ASP.NET system, Sun Microsystems' JavaServer Pages, mod_perl and the Ruby on Rails framework, as they all provide dynamic content to the client from a web server. To more directly compete with the "framework" approach taken by these systems, Zend is working on the Zend Framework - an emerging (as of June 2006) set of PHP building blocks and best practices; other PHP frameworks along the same lines include CakePHP, PRADO and Symfony.

The LAMP architecture has become popular in the Web industry as a way of deploying inexpensive, reliable, scalable, secure web applications. PHP is commonly used as the P in this bundle alongside Linux, Apache and MySQL, although the P can also refer to Python or Perl. PHP can be used with a large number of relational database management systems, runs on all of the most popular web servers and is available for many different operating systems. This flexibility means that PHP has a wide installation base across the Internet; over 19 million Internet domains are currently hosted on servers with PHP installed.[9]

Examples of popular server-side PHP applications include phpBB, WordPress, and MediaWiki.

Command-line scripting
PHP also provides a command line interface SAPI for developing shell and desktop applications, daemons, log parsing, or other system administration tasks. PHP is increasingly used on the command line for tasks that have traditionally been the domain of Perl, Python, awk, or shell scripting.[citation needed]

Client-side GUI applications
PHP provides bindings to GUI libraries such as GTK+ (with PHP-GTK), Qt (with PHP-Qt) and text mode libraries like ncurses in order to facilitate development of a broader range of cross-platform GUI applications.