I am traveling to Europe in May and will have about 5 free hours. Can anyone give me some tips of what I could do with that time? Thanks

There is a little cubicle hotel that looks pretty cool at the airport and a Philly sandwich place.
You could take the train into the city in about twenty minutes and visit Reading Market

I would hop on an Amtrak train from PHL to Newark (EWR) and fly out to my destination at least 3 hours sooner.
If Newark doesn't have the flight you need, check BWI which is also right next to the airport.

We had an 8-hour layover at PHL a few years ago. We took the SEPTA train into the city center (I believe Market East station), got a cheesesteak, walked to the river, then back past Independence Hall and saw the Liberty Bell. Easy way to see a little of Philadelphia, since I'd never been. We were back to the airport in plenty of time to get through security and get ready for our flight.

Will you have carry-on suitcases? That could be an issue. I'm not sure if there are lockers anywhere along that route.

Yes, we will have carry on luggage and there are no lockers at this airport. Tomfuller, we are flying with ff miles, so changing our flights is not an option.