PHL is an easy airport to connect domestic to international. You never leave the airside and even from the furthest set of gates it shouldn't take more than 20 mins to walk to the A gates were BA fly out of. There's also a good BA lounge right by it if you have status / time. If you use the AA App you'll easily be able to get a map of the terminal and it'll tell you the gate your arriving at so you can see how long the walk will be. Of course if you have a delay you're slightly screwed but as long as it is booked as one booking AA / BA will look after you.