The PHPMVC Compat file is a shim that aims at allowing developers a way of migrating across their older PHPMVC (PHP4 code) to Phruts PHP5 code. 

It attempts to do this by injecting back into the application a number of older classes that existed in PHPMVC and setting back up the inheritance. 

It may not be possible to avoid some major refactoring to your application (as PHPMVC didn't seem to have any standardised boot-strapping), but this may assist with getting your application running in PHP5 Phruts quicker.