I know that a PhD is now just used to mean doctorate, so what would a person who has a doctorate in philosophy be called?
Generically, a PhD is a Doctor of Philosophy, not a doctor in philosophy. Thus a PhD in philosophy would a Doctor of Philosophy in Philosophy (as opposed to a PhD in economics, who would be a Doctor of Philosophy in Economics).
Doctor. Just like everyone else with a PhD.
Based on your comments to the question:
PhD is not used to just mean "doctorate." A PhD is a specific type of doctorate. The reason people talk about, say, a PhD in computer science is that that is their actual literal degree -- their diploma says "Doctor of Philosophy" on it, not "Doctor of Computer Science." Especially formally (like for honorifics), you never call yourself a PhD unless that is your actual degree (informally, some US schools hand out ScDs that are equivalent to a PhD, and you might say you have a PhD because it's clearer; however, formally, you do not have a PhD then).
Other than ScD in the US (and D.Phil, which is just PhD in English instead of Latin), other degrees I can think of would never be called a PhD even informally. A DFA is generally honorary, but even if not it's not a research degree. An MD isn't a PhD, even though it's a doctorate. An EdD is not a PhD.
So, a PhD in philosophy would be a PhD. Someone with a PhD in computer science also has a PhD. Things could have been set up so you'd have a D.Math, or a D.Physics, or a D.Comp.Sci., etc.; but that's not how it works, so you really have the actual same degree as people in other fields.
I hold a PhD in philosophy. Long ago I was introduced to a group of non-philosopher cognoscenti this way: "Attention everyone, we now have a PhD squared in the group..." I had never heard that before, but everyone else seemed to know exactly what he meant, and started asking me deliberately silly philosophical questions. A doctor of philosophy of philosophy ... PhD-squared.