We still have a lots of legacy PHP, but its slowly being refactored to Haxe. With Haxe we get a really nice typesystem, and a "faster than Go" compiler. It has pushed our productivity thru the roof.
We still need to use external dependencies tho, as PHP lacks any concurrency in the core language, so we also have a Go API for fetching data concurrently, and also use it as a BI directional socket for the frontend and as a queue server.
Otherwise, the stack is pretty much PHP7 from top to bottom.