Should I write "PhD" or "Ph.D."?

Question pretty self-explanatory. Should the abbreviation of the Latin term philosophiae doctor be written as PhD (no periods) or Ph.D. (with periods)?

Actually both are correct. I could easily find both on my NOAD, and there are plenty of pages on the net where you find it written as "PhD".

The OALD gives an interesting distinction, stating that Ph.D. is especially North American English.

Now, being a non-native speaker, I can only rely on official sources to state who uses what, but there's no doubt that both variants are used.

Lastly, I think there's really no point in distinguishing them as "Philosophiae Doctor" or "Doctor of Philosophy" because it's the same exact expression, although considering the abbreviation, the former is the correct and original long version, the latter is just the English translation.

PhD and Ph.D. are both correct. Canadians tend to omit the periods and those from the U.S. tend to keep them.

I tend to use both 'PhD' and 'Ph.D'. A DPhil is awarded at both Oxford Uni and Sussex Uni in England. All others award PhDs to my knowledge.

It bugs me when people use 'Dr' before their name and then also state the award following it. I feel it should be one or the other.