Sounds like a cool project! If you are not specifically interested in
static graphs, you can also use profilers (XDebug, xhprof) to capture calls
as they happen and display them with KCacheGrind or something similar. That
gives you call graphs which include information like percentage of time
spent in a subgraph and are great for optimization.
Doxygen can also supposedly generate static call graphs, although I don't
know how good it is at it (it's generally not very PHP-friendly).