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]