Sobriety and seatbelt checkpoints have been specifically rules acceptable by the Supreme Court, and to be legal they have to meet certain criteria. Not being even a pretend lawyer, I don't remember what those criteria are. Nonetheless, criteria applicable specifically to motor vehicle checkpoints at specific locations are not applicable to stops at will be any officer on any street.
I think the Philadelphia police department really REALLY needs to re-read the SCOTUS rulings on Terry and Hiibel. Terry clearly established that before a police officer can detain an individual there must be "a reasonable suspicion, based on clearly articulable facts, that a crime has been committed, is being committed, or is about to be committed."
As I posted above, given that open carry in PA (and Philadelphia) is legal with a carry permit, the mere sight of someone carrying a firearm openly does not and cannot give rise automatically to a "reasonable suspicion" of criminal activity. Now, if the officer happened to recognize the individual and know that the individual is a convicted felon and thus a prohibited person ... that's the sort of thing that would give rise to a "reasonable suspicion based on clearly articulable facts."
As to verifying permits -- how does that work? I don't live in PA. I have non-resident permits from both FL and NH (among others), both of which are recognized by PA. So if I am stopped in Philadelphia in the middle of the night for open carrying (to use an EXTREMELY unlikely scenario) ... how long is it going to take the Philadelphia PD to contact anyone in FL or NH to confirm that my permits are valid and current? Suppose I only have FL ... the FL permit is issued by the Department of Agriculture. My guess is that they don't man the phones 24/7, waiting for calls from the Philadelphia PD to verify carry permits. Or what if my permit is from Alaska, Wyoming, Montana, or one of those states?