I have taken extensive PHP applications and re-coded them as a C/C++ module to the PHP language. It worked out successfully. A simple loop in PHP that just does some math and prints our a value, can be 1,000 times faster in C; if a PHP object is created and deleted in the loop, and you do the same in C++, you can get 100,000 times faster. Your speedup will vary according to what it does, of course -- if it is mostly waiting for a database, you won't see much speedup at all.

Contact me if you are looking for someone to do that kind of work.

If I were starting a new web app from scratch today, I would code it in C using Thomas Boutelle's cgic library. Of course, it seems one rarely starts anything from scratch these days.