Of course you can get a PhD in numismatics. It's not easy, but you can get one. Dr. Don Kagin was (apparently) the first person to be awarded one by an American institution, and he is or has served on the advisory boards of several organizatons such as PCGS and Odyssey Marine Exploration.
PhDs in Numismatics are a long-standing feature of classics and ancient history departments, particularly in Europe. Dr. Ilya Prokopov, who assists FORVM with compiling their database of fake ancient coins, obtained his doctorate in numismatics from Sofia University, Bulgaria.
Here in Australia, the only place to do higher study of numismatics is at the Australian Centre for Ancient Numismatic Studies at Macquiarie University, Sydney, where numismatics degrees can be obtained at the Masters level.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis