Open source describes practices in production and development that promote access to the end product's source materials — typically, their source code. Some consider open source a philosophy, others consider it a pragmatic methodology. Before the term open source became widely adopted, developers and producers used a variety of phrases to describe the concept; open source gained hold with the rise of a public, worldwide, computer-network system called the Internet, and the attendant need for massive retooling of the computing source code.

Opening the source code enabled a self-enhancing diversity of production models, communication paths, and interactive communities. Subsequently, a new, three-word phrase "open source software" was born to describe the environment that the new copyright, licensing, domain, and consumer issues created.

The open source model includes the concept of concurrent yet different agendas and differing approaches in production, in contrast with more centralized models of development such as those typically used in commercial software companies. A main principle and practice of open source software development is peer production by bartering and collaboration, with the end-product (and source-material) available at no cost to the public.

See full article on wikipedia at http://en.wikipedia.org/wiki/Open_source.

PHP is a widely-used general-purpose scripting language that is especially suited for Web development and can be embedded into HTML source documents. 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.

PHP was originally created by Rasmus Lerdorf in 1995 and has been in continuous development ever since. The main implementation of PHP is now produced by The PHP Group and serves as the de facto standard for PHP. The latest stable branch release of PHP is PHP 5.

Official PHP website