Yes, most of the built-in functions (over 470) are foreign C/C++ functions installed exactly using the same interface: ph7_create_function();
My first thought on seeing this project was wondering how hard it would be to hook up to nginx, or similar. I don't really "do" PHP, so I'd probably not attempt it myself, but I get the feeling from having written nginx modules in the past that this doesn't look too hard - the PH7 API seems decent enough.
I am using HHVM (Facebook's HipHop PHP w/JIT) with Nginx. that works great... but it requires a 64-bit server. So for smaller servers or embedded systems Ph7 sounds interesting.