You can be stopped and arrested for a DUI in Pennsylvania even if the vehicle is not moving. Picture the example of a vehicle stopped at a red light if the person is driving under the influence the fact that they're stopped at a red light is not certainly going to negate the fact that they were driving under the Influence.
There's other, say, more realistic scenarios where this can come into question. For example, if a person is parked in their car, if they're, say, asleep in their car, the question comes down to is the person in operative control of the vehicle, are they in the driver seat with the key in the ignition, is the car turned on? The more arguments that the prosecution can make that the person was in operative control the more likely it would be deemed that the person is driving under the influence.
In short, yes, a person can definitely be stopped and arrested and charged with a DUI if the vehicle is not moving in Pennsylvania. An experienced Pennsylvania DUI attorney will help you better understand the DUI process and help you best defend against DUI charges.