I asked my father to please remind me of the exact meaning of “Ph.D.”, and when it was appropriate to address someone with a Ph.D. as “Dr. So-and-So”. Dad’s response:
Tricky as tu and vous in French, depends upon his preference. Formally he can be called Professor… or Dr. (the kind that doesn’t help people… teasing!).
Medical doctors can be pompous claiming they are the “real” doctors, though doctor doesn’t mean physician but rather teacher, according to the notion they are licensed (exchanged rights for duties) to employ Death Star technology in the healing of humanity.
Ph.D’s often have to remind them that prior to receiving this technology its (Ph.D.) designers have scrupulously removed most of the buttons that would make medical doctors a danger to themselves or others by friendly fire.
Ph.D. is a doctor of philosophy in a given discipline, indicating he, for example, has published a dissertation and defended it before a jury of peers in a public square (now before a graduate faculty, wives, and hecklers).
When considering the issue of titles, just remember, that in one case you’re dealing with a savant idiot (doctor who’s a dummy) or an idiot savant (dummy who’s a doctor), and that a rose by any other name is still a rose.
In my dad’s case, he defended his dissertation before a graduate faculty, his wife, and me—but I promise I did not heckle. (I was quite a bit younger, though, so I did doodle.)