I find myself in PHL airport often - best you are going to do there is Vino Volo or Sky Asian Bistro. Food is o.k. in both and they have some ambience. There is also a restaurant in the Marriott – think it is called Aviation. Again decent atmosphere and o.k. food.
If you really want a dining experience you will have to go into Philadelphia, which you have the time to do with an 8 hour layover. Rouge on Rittenhouse Square would be a nice spot for lunch.