PHP Selector is a feature enabled by the use of CloudLinux and CageFS. Essentially, both systems allow for isolated enviroments to be created for each hosting user. Allowing them to have control over their own small enviroment. This greatly improves the amount of different options the end user has, which one of these being able to select different PHP versions as well as different modules.
This is a very important peice of software, which shouldnt be overlooked, as many content mangement systems, billing systems and forums require different PHP versions and different modules to work effectively.