In general, PhD refers to a doctor of philosophy which is usually the highest academic degree given by the universities in most of the country. Individual who earned a Doctor of Philosophy can use the title Dr. The title Doctor of philosophy doesn’t necessarily mean the students studied philosophy, it can be other subjects overall.
Generally speaking, in the US, a traditional-age student (say, 20 something) who is accepted to a PhD program at a research university will be funded, that is, tuition is paid for, plus a stipend is provided. In return, the student is typically assigned to be a teaching or research assistant.