PHL is a large base for Republic's USAir flying. If one were to be hired onto the Chautauqua or Shuttle America certificates, however, neither has a PHL base. Hypothetically speaking, of course, as they're not hiring at the moment.
As to which routes are served by each company out of each base, that information is in constant flux in the regional world, so an up-to-date online resource is not something I've seen. However, this link has a handy, mostly updated listing for all of the US regional carriers and their bases:
Airline Pilot Central Carrier Listings