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."
Usually PHP is installed alongside Apache and MySQL in a combination known as AMP, see Installing AMP for details. It is possible to build PHP from source - you might have to if you need a very new version for developing Moodle - however, it is quite challenging. PHP itself has many dependencies that you will need to obtain and build, some of which are also tricky to build from source (e.g. GD).
Minimum version requirements
The minimum version of PHP required for Moodle is listed in the requirements section of the installation instructions