This looks like just a stub; I don't see a sign of a compiler, interpreter, runtime, or any embedded or dynamic linking.

What exactly does it do besides evaluating PHP booleans from Ruby?

Please see the ext directory:

http://github.com/tenderlove/phuby/tree/4f198354c537586aa167...

and extconf.rb:

http://github.com/tenderlove/phuby/blob/4f198354c537586aa167...

It is quite embedded and dynamically linked.

Not a stub, a bridge. As you can see in the readme, there IS a php runtime embedded in ruby. So that covers your redundant requests for interpreter and runtime.

Compiler? Why?

Embedded and dynamic linking? Does that even make sense in the PHP world? Sounds nonsensical to me, but I've avoided PHP until now.