When to arrive at PHL for 6 AM Flight?

We are headed to Puerto Vallarta via a plane change/layover in Chicago from Philadelphia airport. Our first flight to Chicago departs at 6 AM. At what time should we be at the Philadelphia airport? I know that for later flights, we are required to get there two hours early but I'm not sure what time airline representatives are actually present in the morning. Also, does anyone know if we should go to the international terminal vs regular terminal? I'm thinking international because our ultimate destination is Puerto Vallarta, MX but then again, the initial flight is to Chicago. Is there somewhere online that I could go to research this question? Any guidance is greatly appreciated. I am flying US Airways to Chicago and United to PV. Full flight plan was scheduled via one confirmation.

You check in at the US Airways domestic counter in PHL. They'll give you your boarding pass for both the US Airways and United flights. I'm guessing you want your bags to be checked all the way to PV, right?

The US Airways website explains how early you have to arrive for your flight. I think it's around two hours prior especially if you've got luggage to check in. The US Airways ticket counter agents need to be there to hand out boarding passes for the early AM flights. Otherwise, you wouldn't be able to get thru security.

In Chicago, you just head to the international terminal to your United gate and hope your luggage will make the transfer as well..

Min

I would second the "two hours" suggestion. I believe luggage on international flights goes through some additional screening; this would happen in PHL as by the time you get to ORD there probably wouldn't be enough time for it to happen and your bags can just be transferred to your connecting flight.

Your first flight is a domestic flight, and would leave PHL through the Domestic terminal. There are plenty of people on that flight who are not going beyond ORD or are going to other destinations and for them their trip is purely domestic.