Actually xptr_eval works exactly the same as xpath_eval and doesn't return an integer as suggested. I haven't tried anything with it and PHP (4.3.0 for me now) doesn't seem to be any help either. Even simple examples (from http://www.zvon.org/xxl/XPointerTutorial/Output/index.html) don't work, just because half of XPointer is not implemented. XPointer expressions like /1/1 do work.
Let's face it, this is all experimental, folks!
xpath_register_ns