Generally speaking, in computationally intensive projects, code doing the same steps is 100 times faster in C (or C++ for that matter) compared to PHP. Optimizing your C will give another 2-10 times increase, depending on the time, effort and knowledge you put in.
The point is that PHP is interpreted, and C runs, loosely speaking, almost directly on your cpu. If you really want to get the most out of it, in C you can go down the SSE1/2/3/4 road.
You could of course use or write a library (or call it "extension") for PHP in C, which in my experience is a good match to achieve high speed at the right points while keeping the benefits of PHP.