I don't know of a way to detect code that is completely unused, that may be beyond the abilities of all the tools out there. But in terms of the tools out there, hit https://phpqa.io/ for a good rundown of them.
So far, one of my favorites in phploc which tears apart your code from an Object Oriented perspective and gives you details on how many classes vs how many functions vs how many tests vs average loc per function vs Cyclomatic Complexity.
My next favorite is phpcpd which is the "PHP Copy-Paste Detector". It tokenizes your entire codebase, looks for common signatures, and gives you a list of files with line numbers.
There are lots of other tools on that page, choose the ones that are useful to you.
We're actively using these tools in web2project and in the two years since we forked from dotProject, we've dropped about 35% of the codebase from refactoring, eliminating duplication (originally 12%, now about 2.5%), and generally structuring things better. And that's counting our 15k+ lines of Unit Tests. :)
