PHL is a particularly obnoxious place to try and get gas. It completely depends on where you're coming from as you're approaching the airport. If you're coming north on I-95, I would get off at Essington (exit 9) and go south on 420 to the first light, then turn right on 291. There is a gas station right there, and you can get back on 291 going north and follow it right to the airport.

If you're coming south on I-95 from the east/northeastern part of Philly, or I-76 from the western half of Philadelphia, things are much tougher. There used to be a gas station right on Island Ave. just north of the airport, but the last time I went to use it, it had closed. A Google search produces these options, none of which are good. No idea of their hours. Frankly, I'd just get gas the night before, as others have stated.