There's no catch. International fares generally allow you to have up to 24 hrs to connect at any point along the itinerary. In this case the PLS-PHL fare is cheaper than PLS-BOS...it just so happens that they allow connections in BOS.
If you need to go to PHL the day after then by all means book it and don't look back. In fact, for a party of 3 to save $900, I'd be tempted to book all 3 the same way, then have the other 2 get sick in BOS and not feel up to flying to PHL
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