Random number generation speed
In your situation, I'd keep the random generator server side. Perhaps look into other languages to put together a little C programme if you're really focused on speed and server load (PHP is resource expensive). But sticking to the tags, again: stick to php - after all, quite a lot of it is C anyways...
The best bet is to write a loop in each and run it to generate a large number of random numbers and show you how long it took. That way you will know which runs better for your particular setup.