I have eaten many a lunch or after-flight light meal at the Penrose Diner, about 10 min from PHL over the Penrose Bridge. It is open 24/7 with the usual diner and booths. Quite reasonable prices and big big menu. Plenty of the Philly cops and shift workers eat there 24/7 too. The Melrose Diner has about the same food for slightly more money but more difficult to get to. Do a google for directions and info.
MisterNice