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)