Being asked today to provide a biography of three of my colleagues and myself (for inclusion in a paper), it sure hits me how the master's degree and PhD degree are written in many different ways.
Looking it up, I concluded the following...
For the master's degree, one can write correctly both (and only) MS and M.S. (Magister Scientiae) if one likes Latin and abbreviations. Or, one can stick to just writing master's degree (which I adopt as a standard from now on).
For the PhD degree, one can write correctly both Ph.D. and PhD (Philosophiae Doctor), but unfortunately is obliged to use a Latin abbreviation. (It's not that I do not like Latin, but rather that I'm fond of clarity.) To the best of my knowledge, there is no internationally accepted way to write a PhD title in full. (Hope someone invents it by the time I obtain one, but I doubt it. Please tell me in reply to this post if you know one.)