I have a kid that goes to college near there and started looking into it. I've also done my share of subtances, including the really warm and pleasant ones.
I don't know much about street spots, but I do know that some downtown areas are now totally off limits, including the Love Park area. It's actually listed in some kind of criminal code. If your search for "legal skateboarding Philadelphia" or something like that you can find the legal info and the exact places where it is specifically prohibited.
FDR skatepark is the big standout in the area. It's predominantly big bowl type transition skating, but there is a small amount of DIY street stuff as well. The place was built by locals and can feel pretty territorial. Someone might want to check it out earlier in the day when it seems to be less busy, especially if you want to skate a lot aren't good enough to earn some respect quickly.
There are some other skateparks that show up on Google Maps, but nothing that I've seen was particulary nice or impressive.
My advice to anyone looking for good streets spots would be to hang around FDR and try to make some local contacts. Like I said, FDR can get a bit rough at times, but with the right background and attitude you shouldn't feel too out of place. That's gonna be the doorway to the Philly scene.
Just don't end up getting yourself into trouble, which might be a real possibility if you enter that world. Certain kinds of places and people have a way of stirring up certain kinds of feelings.