Note: You are currently viewing documentation for Moodle 2.2. Up-to-date documentation for the latest stable version is available here: PHP.
PHP is the scripting language in which Moodle is developed. PHP does not stand alone, it is tightly integrated with a suitable HTTP Server. Most people use the Apache HTTP Server for this purpose although a number of others are supported.
PHP describes itself as "a widely-used general-purpose scripting language that is especially suited for Web development and can be embedded into HTML."