Before the industry revolution (which happened only a few hundred years ago), philosophy and science went together. PhD might be a conventional use of that philosophy concept.
This is a medieval tradition only. The scientific degree of PhD is a certificate of entry into the "guild" of scientists. Sometime this is not related to philosophy, as well as to science in general.
In Russia, for example, the first level is called "Candidate of Science," and science is named. For example, Candidate of Sociological Sciences, Candidate of Chemical Sciences, Candidate of Technical Sciences, etc.
But this does not change the ritual nature of a scientific degree.