PHP is a widely-used, general-purpose scripting language that is especially suited for web development. Use this tag when you are dealing with a PHP script, installing PHP, or configuring a PHP installation.
Adapted from Wikipedia
PHP is a server-side scripting language designed for web development but also used as a general-purpose programming language. PHP code may be embedded into HTML code, or it can be used in combination with various web template systems, web content management system and web frameworks. PHP code is usually processed by a PHP interpreter implemented as a module in the web server or as a Common Gateway Interface (CGI) executable. The web server combines the results of the interpreted and executed PHP code, which may be any type of data, including images, with the generated web page. PHP code may also be executed with a command-line interface (CLI) and can be used to implement standalone graphical applications.
PHP's official website is: http://php.net/.