PhD is an abbreviation for Philosophiae Doctor, Latin. Usually abbreviations are initials, but PhD uses the first two letters of Philosophiae. The reason for the use of two letters is most likely due to the fact that the ph of the Latin philosophiae pronounces a consonant, which is very different from the p pronunciation.
philosophiae is the property of the noun philosophia. The case is equivalent to an adjective in English, so philosophiae doctor is doctor of philosophy in English, where philosophy is an English noun, corresponding to the Latin philosophia.
According to the English Doctor of Philosophy, the abbreviation is dphil, and the addition of il may be for ease of pronunciation (or it is possible that dp and dph are too recognizable).
Because PHD has no vowels, it can only be pronounced by three consonant letters (four syllables).
According to the current English language, the abbreviation should be popular from Doctor of Philosophy, but PhD has been around for a long time, and a long, long time ago, it was fashionable in Latin, not in English. So dphil is just popular in British English, and PHD is the mainstream internationally.
According to the European language abbreviation Xi, the abbreviation should be expressed with a period, that is, the orthodox abbreviation of philosophiae doctor is phd.。Such a style is indeed used in American English (PhD for Canada). Similarly, dphil should be dphil. 。
Philosophy is generally translated as "philosophy", but philosophy means love of wisdom in ancient Greek.
The British doctoral system appeared very late, at the beginning of the twentieth century, so Newton, Hamilton, and Rayleigh could be knights, but none of them were doctors.