EDIT: Someone downvoted me, not sure why, but I am genuinely curious what the senior engineer does?
It's interesting to note though, that Facebook does uses PHP and C++ extensively. And Facebook maintains a large a number of open source PHP and C++ projects. So I suppose that some Facebook engineers are more involved with PHP and some more involved with C++.
But more importantly, I don't think Facebook engineers are bound to a technology. They think about how to solve problems. If PHP is the best tool to solve a problem, they'll use that. If it's C++, they'll use that instead.
You're right about them using the right tool for the job - their entire HipHop etc. (whatever it is called now) creations to convert PHP to C++ and run a C++ web server are a good indication of this. Good point!
[1] https://www.facebook.com/notes/protect-the-graph/supporting-...