In PHL you'll be leaving from A-West on US, which has a new US Airways club upstairs as you're going down the hall to the A-West gates. If you're in Envoy (business class) the Envoy lounge is to the left, and the US Airways/*G lounge is to the right when you get to the entranceway. (I know you said all travel is in Y, but just in case. Maybe they'll send you to the Envoy lounge anyway...who knows?)

There are other lounges in PHL, but that's the nicest (especially if you're Envoy) and the closest to your gates.

On return to the US you won't have access to anything in PHL since that's your endpoint and you're taking a stopover. If you were continuing, you'd have access to any lounge since it's an international connection, but sometimes you have to explain that rule to people.