Is that the rest of some technical feature or set of features, or just that most college graduates can be expected to have some minimal competency with C++ coupled with the fact that the PHP interpreter was written in C? I am inclined to think this is more a pragmatic decision than a technically motivated one.
"[...] I started my first piece of code transforming PHP into C++. The languages are fairly similar syntactically and C++ drastically outperforms PHP when it comes to both CPU and memory usage."
It sounds to me like they wrote a PHP compiler that leverages GCC's optimizer. I am not seeing the technical motivation to use C++ for that; it would have been just as good to just produce GIMPLE. It looks like C++ was just chosen because it is what the developers knew, which is not all that far off from what I said.
(Edit: it is also worth pointing out that the motivation for this tool is to avoid having to write C++ code)