PHP is a server-side scripting language designed for web development but also used as a general-purpose programming language. As of January 2013, PHP was installed on more than 240 million websites (39% of those sampled) and 2.1 million web servers.^[4]Originally created by Rasmus Lerdorf in 1994,^[5] the reference implementation of PHP (powered by the Zend Engine) is now produced by The PHP Group.^[6] While PHP originally stood for Personal Home Page,^[5] it now stands for PHP: Hypertext Preprocessor, which is a recursive backronym.^[7]

PHP code can be simply mixed with HTML code, or it can be used in combination with various templating engines and web frameworks. PHP code is usually processed by a PHP interpreter, which is usually implemented as a web server's native module or a Common Gateway Interface (CGI) executable. After the PHP code is interpreted and executed, the web server sends resulting output to its client, usually in form of a part of the generated web page; for example, PHP code can generate a web page's HTML code, an image, or some other data. PHP has also evolved to include a command-line interface (CLI) capability and can be used instandalone graphical applications.^[8]