Eric has made very valid points on the value of a PHd in museum
studies. First of all, it's difficult to find a university that
would offer it. The one best known is the U. of Leicester in the U.K.
Most people earn their doctorate in another specific discipline such
as art history, history, or one of the sciences. Some then design
the course of study to relate to museums. If you want to be a
conservation scientist, there's a fairly new PHd program at the
U. of Delaware. Lynne TEather who is director of Museum Studies
at the U. of Toronto has a PHd from  the U. of Leicester. She is on
Museum-L. What is it you want to do in a museum? What kind of a
position are you anticipating? That will determine more or less
your educational requirements.  Good luck!