Local NYC taxis will probably not take you to Philadelphia and it would be very expensive. The best would be Megabus (stop is at 31st St.) or Amtrak train both from Penn Station. You can get to Penn Station by taking the NYAS bus from your terminal or Airtrain to Jamaica Station, transfer to LIRR or E subway to Penn Station. Amtrak has frequent departures to DC which stops in Philly. You can also check at the Ground Transportation counter when you arrive for possible shuttle service.
Local NYC taxis will