In physics photon counting refers to the ability to count a single photon or to distinguish between N and N+1 photons. Your photoresistor will be able to see differences in light intensity only if they are so large that they correspond to billions (or more) of photons.
When counting photons usually high repetition rate pulsed lasers are used, such that the electronics has to count at various MHz. What is the maximal counting speed of Arduino UNO and DUE?