Nah, C and php are lame, better go on pure assembly or plain binary. Why not just a hardware device? FPGAs are cheap
Just joking.
The main difference is that php+mysql is very easy to be scaled and is completely painless. C in exchange is not. If you start from there you can make a better decision than speed of C and php.If you use zend or turck encoded php it will be the same fast as C (don't forget that C loads lots of system libraryes on each run). It's not A is faster than B, it's more like program design A is faster than program design B (whatever it is written on). You can have slow program running C. Windows is a perfect example