This thread, A US guide to PHL (the airport), should be helpful.

PHL is also my home airport, so I'm not positive about this, but after you claim your luggage you then clear customs. At that point you are landside. Follow the signs to recheck your luggage. You will now have to go back through security to get to C. (I wouldn't try walking from A to C outside of security...you can't do it inside the airport and the only option outside involves crazy Philly drivers.) (And the fact that you have been landside means that any liquids you kept from the plane or from duty-free must be checked.)

It may be helpful for a mod to move this to the US forum for additional detail.