AirTrain to EWR train station, then Amtrak to Philly or NJ Transit to Trenton + SEPTA to Philly (cheaper, more annoying).
Amtrak is a single ticket (inc. AirTrain fare). $50 or more total each way.
http://panynj.gov/airtrainnewark/
http://www.amtrak.com/timetable/july07/W02.pdf