Many PHP applications funnel server requests into a single (or few) PHP source file that sets up the environment and configuration for the application, manages sessions and caching, and invokes the dispatcher for their MVC framework. They can do more, but their main job is to take care of the consistent needs of every page of a web application.
In our Blueprint for PHP Applications, we will have a core bootstrapper that receives all dynamic requests for an application and applies a template for application behavior that we can later extend. It will allow us to later customize the functionality for each unique application.