Protecting the Arduino is moot, since this circuit as drawn in replies 12 and 13 will never turn off.
With 20V on the PNP emitter, you'd want something like a 22V zener and a 1k resistor in series between the Arduino output and the PNP base. Then a 1k resistor from the PNP base to 20V.
But that's a bad idea, too, because it is so fiddly. The Zener is going to be not quite biased on when the Arduino is at 5V. It just seems simpler to add an NPN transistor.
