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."
Minimum version requirements
The minimum version of PHP required for Moodle is listed in the requirements section of the installation instructions