On the official announcement they describe what the tool is and what it can do for you:
Rasmus Lerdorf took the opportunity to play with this new tool and run some benchmarks on its performance in a more real situation than the form example Facebook gave:
The real question here is what is this runtime xml validation going to cost you. [...] Note that to build XHP you will need flex 2.5.35 which most distros won't have installed by default. Grab the flex tarball and ./configure && make install it. Then you are ready to go.
He created a simple class (a "singleton") and ran some benchmarks against it with Siege. I won't share the results of those benchmarks here, though - you'll need to visit Rasmus' site for those (but here's a hint, the future's not bright).