John,
To piggy back some on what kHz said, PhD programs do have some degree of flexibility. With that in mind, you will want to have some deep meaningful conversation with the professors of whatever university you plan to attend. Make sure that the types of things they are working on are the types of things you are interested in. I know people who entered into PhD programs wanted to do "XYZ" and would up just extending their advisor's work. That's not to say that it will happen to you, but just make sure that you and the potential school you're looking at are on the same page in terms of goals.
Another thing to keep in mind is your goals once you get your PhD. Do you want to teach at the university level? What type of university do what to teach at (i.e. an R1 university where research is first and foremost or a university where research is not as important as teaching the students). Do you want to work for a Think Tank, or a consulting firm?
Also, like others already have on this board, I would advise against an online PhD programs. I'm not saying that all of them are bad, but when it's time to get a job, employers (especially if you intend to get into academia) want to know with whom did you study, what type of research did you do, what types of projects were you affiliated with during your time in school, etc. This is a tougher question to answer if you got your degree online.
Whatever program you decide to enter keep in mind a few things about the PhD:
1. At some point you may have to go fulltime (if not go fulltime from the start) to finish the program in a timely manner
2. Expect it take 4 or more (emphasis on "or more") years to complete
3. The toughest part is not getting through the classes but actually completing and defending the dissertation.
I hope this helps.
--JW