Possibly neither; it depends on what you want to do.
Both PHP and Python are scripting languages, and are therefore not suited for high-performance numerical routines (which most image processing requires). However, they both have a number of image-processing libraries available for them, the innards of which are probably written in C. These will be fast.
If these libraries do what you want, then fine. If you need to so something custom, then you're probably better off with C or C++ (or Pascal, or whatever) if speed of execution is of concern.