As noted already, Doxygen can generate call graphs.
Xdebug can generate function traces. These differ from doxygen's graphs in that they are generated from runtime code, whereas doxygen is generated statically. I don't know of any tools that can turn the function traces into a visual representation, although it shouldn't be that hard to do.
There is also the bytekit extension, which primary function is to show the bytecode that php source code will generate. It can show this in a graph, that is essentially a callgraph.
