Strictly speaking a PhD isn't a PhD "in" something. It is simply a PhD (a doctorate in philosophy).

Obviously philosophy isn't that informative a term, so people try to be helpful by adding what field or topic their actual research was about.

Some vocational/applied higher degrees in specific fields do have specific initials to differentiate them from a "generic" PhD (for example, the EngD).

Apologies for ++pedantry :)

This is also why in non-English speaking countries, this title usually does not have any relation to philosophy. It is "Doctorate in something".

Sure. But it was the commenter writing the misunderstood joke that said 'PhD in' (just omitted the next word), I was just explaining that.

> people try to be helpful by adding what field or topic their actual research was about

Right, and a common way to do that is to say 'PhD in blah', or 'on bazzing foobars for blah' as though 'PhD' referred specifically to their thesis.