Open carry is 100% legal in PA because there is no law against it. In other words, you will not find a law or statute saying "open carry is legal". Since there is no law against it, open carry is legal in all of PA without any license or permit. However, only in Philly is a license to carry firearms (LTCF) needed in order to open carry. There is a law on this; PACS 6108.
But, lacking a LTCF you cannot transport a firearm (handgun) to a place where you desire to OC. In other words, you can strap on your handgun openly (not concealed) and walk out your front door. If you get into a vehicle, you need a LTCF.