Inverting a dependency list

I recall, years ago, having a set of utilities that would build a call tree from application code written in C. This was useful for figuring out dependencies in code that someone else had written. I would like to do something similar with a large PHP application I am now maintaining, but with a slightly different emphasis. This application includes several library files which contain more than 400 function declarations. I need to determine how many of those functions are actually used by the application and which can be culled from the code base. Is there an easy way to determine which of them are called somewhere and then work through the call tree to identify the orphans? Bob McConnell