bitburgr's info is correct. You will clear immigration first, then claim your bags, then clear customs. As you exit the customs area (you're now landside), transit passengers are directed to the bag re-check point. From there, you re-enter security and get routed back into Terminal A. It's about a 5-10 minute walk to Terminal C.

This post in bitburgr's excellent guide to PHL is even more comprehensive.