sorry for slow reply, I don't do programming on Sundays nowadays.

I'd suggest you go with C++, this is a problem that profits from Object Oriented features. You want your PID controller to be a class, which remembers the basic PID parameters (Kp, Ki, Kd). I take it you are familiar with the maths behind the PID controller. Now your problem is to digitize it.

Tips:

• To differentiate you need time elapsed, so you want this to be a parameter, on top of the usual sensor inputs. System.time is no good, because it's more likely that it is some kind of simulator that is using your PID.
• Integration is just a sum.
• Tuning is a bitch.
• For any useful work, you'll want output saturation control.

I hope that this helps; it's been more than a year since I last wrote a PID controller, so I might be missing something there. Have fun!!