I think the really hard stuff isn't done via PHP. Seems to me they use PHP as the front end because they want to focus on their really valuable technology - their C code.
PHP is used as a small layer that enables talking to our C code from javascript. We have a custom (Protocol Buffer-like) protocol to manage our RPC, the PHP embeds a native module that implements that protocol and exposes a webservice to which our Javascript code can talk in order to provide some valuable user-experience on top of our C-written technologies.
Nowadays there is so little intelligence in the PHP that we only consider it as a pass-through layer.
