All of PHL is connected airside, no need to clear security. Your flight from MCO will most likely arrive at concourse B or C and then you'll just walk to concourse A. It's not like LHR where terminals aren't connected. As said above, no reclearing security, no collecting bags or anything.

If you miss your connection at PHL, then you would go to AA for help rebooking. It would be on them (as the reason for missing the connection) to help you. One caution, however, in the US, there is nothing like EC261/UK261 protecting you when flying a US airline to the EU (UK261 would cover you if flying a US airline from the UK/EU), so if for some reason you are delayed over night, unless it was for something like a mechanical delay, then AA will not provide a hotel. You would be responsible for that.