Subclassing website

Hi all! I want to subclass a large website mainly written in PHP. The sublassing is useful for testing when you want to test a change, a bugfix or a new design, and only a few pages change. The subclass would consist of an almost empty representation of the website containing only the changed files, thus allowing testing without harming the ordinary website. Is it possible ro postprocess the output of PHP for a specific page (e.g. for changing all links dynamically adding a prefix to a path, if that file does exist in the website subclass)? Thankful for hints/pointers on this! Maybe there is already a module for this? Thanks! /Henning