Dr. Free-Ride answers the question about what credentials are needed to become a philosopher. It's interesting to note, actually, that in library schools - places where one goes to study to become a librarian - folks might have a PhD in just about anything. From education to computer science to anything starting with information (systems/studies/science/technology/management) to economics or business or psychology.... In fact, most of the courses you need to take to become a librarian are taught by adjuncts who are practitioners.
Funny thing is that ALA's accreditation committee wants to change that and the library schools are not amused 🙂
(oops had to change the title - I goofed)