PHP is a general-purpose scripting language that is especially suited to server-side web development where PHP generally runs on a web server. Any PHP code in a requested file is executed by the PHP runtime, usually to create dynamic web page content or dynamic images used on Web sites or elsewhere. It can also be used for command-line scripting and client-side graphical user interface (GUI) applications. PHP can be deployed on most Web servers, many operating systems and platforms, and can be used with many relational database management systems (RDBMS). It is available free of charge, and the PHP Group provides the complete source code for users to build, customize and extend for their own use.
PHP acts primarily as a filter, taking input from a file or stream containing text and/or PHP instructions and outputting another stream of data; most commonly the output will be HTML. Since PHP 4, the PHP parser compiles input to produce bytecode for processing by the Zend Engine, giving improved performance over its interpreter predecessor.
Originally designed to create dynamic Web pages, PHP now focuses mainly on server-side scripting, and it is similar to other server-side scripting languages that provide dynamic content from a Web server to a client, such as Microsoft’s ASP.NET, Sun Microsystems’ JavaServer Pages, and mod_perl. PHP has also attracted the development of many frameworks that provide building blocks and a design structure to promote rapid application development (RAD). Some of these include CakePHP, Symfony, CodeIgniter, Yii Framework, and Zend Framework, offering features similar to other web application frameworks.
The LAMP architecture has 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 or some mix of the three. Similar packages are also available for Windows and OS X, then called WAMP and MAMP, with the first letter standing for the respective operating system.
As of April 2007, over 20 million Internet domains had Web services hosted on servers with PHP installed and mod_php was recorded as the most popular Apache HTTP Server module. PHP is used as the server-side programming language on 75% of all Web sites. Web content management systems written in PHP include MediaWiki, Joomla, eZ Publish, SilverStripe, WordPress, Drupal and Moodle. All Web sites created using these tools are written in PHP, including the user-facing portion of Wikipedia, Facebook, and Digg.