Brad, I AM a landlord in PA. Just start with the simple stuff - READ the current lease. This will spell out CLEARLY if you're current presence there is "allowed" or not. If there is no clause that directly prohibits it, then that's ok. In any case it is not YOU that is potentially violating whatever lease is in place right now - that would be your roommate, not you.
Anyway, bottom line is if you two have been good tenants (clean, quiet, paying on time), it is pretty likely that the landlord would love to get your call and simply keep you in place. Sure, you may be required to fill out an application and credit check but that's no big deal.
What do YOU want? Do you want to stay, if the price is right? There may be some really good opportunities available to you with vacancies in your area that you may not be aware of ... So if moving wouldn't be too horrible, then think about your next options. Check out Craigslist to see what's listed there. Also you can post your OWN AD so that prospective landlords can find you (we do that).
Step one is find out the terms of your roommates lease. Are you permitted to be there or is your presence breaking the lease? Second, check out the link below regarding the PA Landlord Tenant Handbook. Your rights will be spelled out there. Finally, chances are the landlord will be glad not to have a vacancy. You'll likely have to sign a new lease and pass a credit check.
You will need to talk to the landlord if you are not on the lease. Chances are they might let you assume the balance of the lease (which would let your roommate off the hook). But depending on the landlord, you might have to fill out an application and undergo a background and financial check before you would be able to lease.
As you are not on the lease, you would probably have no rights - or obligations - associated with being on a lease.