Is it possible to work full time and complete a PhD?
I am doing that right now.  I have a full time job and am working on a PhD in Computer Science.  It is definitely possible, but has been the hardest experience of my life.  I am past the hardest part and am wrapping up my first publication.  I've also been at it for 3 years (already had my masters degree), so it's taken me much longer to get to this point than it would be for a full time student.

It has been extremely stressful and you can kiss your life outside of work+school goodbye.  You will also need to be very good with time management and be aware that over the course of several years, there will be life events that pull you away from school.

When I was deciding whether or not to do it, I vastly underestimated the amount of time and energy required.  I'm very glad I stuck with it so far, but I have to say, if I had an accurate picture of the workload I probably would have opted against it.

You need a huge amount of dedication to the goal in order to pull this off.  If you have only a casual interest in the degree, then you will probably fail.  I think I remember somewhere that the graduation rate for PhD's is around 50/50.  Add a full time job on that and the odds are against you.  But it is absolutely possible to overcome that with enough effort.