PHP is a general-purpose scripting language that is especially suited for web development . It is the fourth most popular computer programming language, ranking behind Java , C , and Visual Basic . ^[23] PHP generally runs on a web server , taking PHP code as its input and creating web pages as output. It can also be used for command-line scripting and client-side 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 . 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. ^[5]

PHP primarily acts as a filter ^[24] , taking input from a file or stream containing text and/or PHP instructions and outputs another stream of data; most commonly the output will be HTML. From PHP 4, the PHP parser compiles input to produce bytecode for processing by the Zend Engine , giving improved performance over its interpreter predecessor. ^[25]

Originally designed to create dynamic web pages, PHP's principal focus is server-side scripting ^[26] , 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 system, Sun Microsystems ' JavaServer Pages ^[27] , 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 , PRADO , Symfony 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 can also refer to Python or Perl .

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. ^[6]