Well... they wrote their own PHP-to-C++ compiler (which is both insane and incredibly impressive), then their own PHP tracing JIT compiler, and their own PHP type checker....
At what point does it become their own language?
Also a good amount of their backend services use better languages like Haskell, D, Erlang, and whatever else they need.
There's a reason the combination of type checking, proper containers and generics, inline XML, and asynchronous processing is all called Hack, and the runtime is called HHVM. At this point, new code using all these features only vaguely resembles PHP, but it's far more robust and maintainable, and vastly more performant.
At what point does it become their own language?
Also a good amount of their backend services use better languages like Haskell, D, Erlang, and whatever else they need.