Is 1h layover at PHL enough to connect to the next flight?

Promoting some comments to an answer

• Is it a sensible connection to do on two tickets? No

• Is it a sensible connection on a single ticket? Depends on the traveller

If it is all on one ticket, then the airline in question will rebook you if you miss the connection for whatever reason. They will rebook you onto the next available flight. Not the word available there, you might be in for a wait at busy times. They may well not rebook you onto another carrier. Depending on the cause of the delay, in the USA, you might well have to fund your own hotels and food during your wait. (Travel Insurance may cover it, it's normally well worth getting one which covers delays + missed connections. EU261 is much better than US regulations for traveller support for delay, so since it doesn't cover you, travel insurance is a big help)

Next thing to consider - when might that next available flight be? If there are flights every 90 minutes, probably not that long. If flights are only once a day, or worse once a week, that's a long wait! If you're flying at a very busy time like a holiday, you need to factor in potentially having to wait for 2-3 planes. How much of an impact will that have on your plans?

Another thing to consider - are there earlier flights to the connection point? And could you get one? If the flight to the connection is only once a day, not much you can do here. If they're every 90 minutes and you don't have any major plans for the time before, try to switch onto an earlier flight, or otherwise go to the airport early and try to standby for the earlier one on the grounds of a tight connection

Next up - any customs / immigration / security at the connection? On a purely domestic itinerary, the first two are a no, but I'm trying to keep this general. At PHL, I believe that all the gates are connected airside, so you won't need to go through security again. At somewhere like JFK, if you're changing terminals you'd have to re-clear security, so you'd have no hope.

Now, check on-time performance for the flight to the connection. If it looks fine, you should be OK. If it looks routinely bad, give up!

Now, with the on-time data, check typical gates for both flights, and check a terminal map. If they're close, you're ok. At small airports, you'd be OK. At airports laid out well for transfer (eg CLT), you're probably OK. At somewhere big like PHL, it could take a while. If it's looking bad, practice running with all your stuff! It won't take a very big delay to mean you have to run. As a % of the flight time, pretty much any delay to the inbound will cut it tight

Finally, how stressed do you get? Will you spend the whole of the flight there stressing about a tight connection? Will you lie awake the night before stressing about it? Or will you step off the inbound in a zen-like state, go "oh, I missed it, shucks", and wander over to a customer service desk in a relaxed manner to ask nicely for rebooking? If you're a stressed and/or nervous traveller, don't do it!

At the end of that, you should have a good answer on if the connection is OK for you or not. I've done a 45 minute connection in CLT before, made it, but regretted it as I hadn't properly thought through the impact of missing it when booking and stressed a bit. Yesterday I did a 1 hour connection through DOH (Doha), with a delayed inbound connection, and made it to the gate during the final call / gate closing stage. That was fine, as I knew there was another flight a little later, and a nice lounge to wait in if I hadn't made it, and as it turned out I wasn't even the last one on the plane.

Tight connections on one ticket is a personal preference thing, think it through then do what works best for you!

Alternately, if connecting between two different tickets, you need to allow enough time for any possible delays, to collect and re-check bags if the two tickets didn't allow interlined luggage, and enough time that your travel insurance (if any) will cover you for a missed connection. 3+ hours should be the minimum, longer if you're self-insuring. Unusually, AA actually have a somewhat hidden published policy on OneWorld<->AA missed connections on two tickets. It used to be on a poorly linked-to page, now it's buried in a PDF on page 11, but even then, I think you'd get some resistance on a 1-hour connection between two tickets