I would either fly with it as a carry-on or ship it via FedEx (packed well). That said, my concern would be that you'd be asked to check the dress for your flight to PHL. Any chance of just leaving directly from PHL? I think I'd probably opt to ship it to someone at your destination, quite early, and make arrangements for them to have it pressed. Most bridal shops should be able to do to that, as should most cleaners.
If you don't know anyone at your destination, but are working with a wedding coordinator, they should be able to handle that.