This depends on the type of doctorate you are referring to, such as the abbreviation for medical physicians such as a Doctor of Osteopathic Medicine (D.O.) or Doctor of Medicine (M.D.), research doctors such as a doctor of philosophy (PhD) or a doctor of education (EdD). Still as such the above mentioned in the plural would be as follows.
Doctors of Medicine (MDs)
Doctors of Osteopathic Medicine (DOs)
Doctors of Philosophy (PhDs)
Doctors of Education (EdDs)
The above are most common within the United States.
As you say, it is doctors of philosophy, or in abbreviated form, PhD's.
M.D.'s
"Medical" is usually used as an adjective, and therefore in its usual use does not have a plural.When the phrase "medical test" is shortened to "medical" and used as a noun, the plural is medicals.
The possessive form of the plural noun doctors is doctors'.Example: The doctors' offices are in building two.
it is Mister's. If it was abbreviated Mr's.
The plural form of the noun doctor is doctors.The plural possessive form is doctors'.Example: The doctors' offices are in building two.
The correct plural form is "Doctors of Philosophy."
The possessive form for the plural noun doctors is doctors'.Example: The doctors' lounge is on the third floor.
Doctors or Drs.
Mrs is "madame" in French. The abbreviation is 'mme'. The (irregular) plural is "mesdames", abbreviated 'mmes'
Mademoiselle is abbreviated Mlle (often written Mlle). (not "melle") The plural form is Mesdemoiselles, abbreviated as Mlles (also written Mlles).
The plural form of Doctor of Philosophy can be abbreviated as either Ph.D's or PhD's.