I'm not so clear about this part: do you mean PhD in Distance learning or PhD by distance learning? If its the former, then I guess you would go through a school eg. School of Education in XYZ University to do a phd in distance learning. If its the latter, yes its possible :-) first check out which schools offer phd by distance learning: for example the School of Distance Education in ABC University; next check out what's offered; type of phd, supervisors and how they run their distance education. I went through distance learning; the university had a very effective methodology/ pedagogy with exams running and compulsory on-campus time etc.
Cheers, satchi.