US, Europe or elsewhere? What kind of interview?
I'll assume it is physics.
If you did research before, expect questions about that. What did you do, can you put your research topic in a larger context? What is related work done there? A presentation of your work is common.
Ask questions about the PhD position, unless it is something you can look up (you should do that first). What exactly will be the research topic? If applicable: Does the institute have a setup already, or do you start from scratch? Who is/was doing something similar at the same institute? If relevant, what about collaborations with other institutes? Travel?
If it is an experimental position, you'll probably get a lab tour, that is a good opportunity to ask questions as well.
Financial stuff: Often the universities have fixed rules, if you can't find them on their websites ask about it. How much do you earn, how much do you have to pay?
If you have relevant travel costs for the interview, check in advance if the university covers that. Probably yes, but I don't know if it is like that everywhere.
If you find some time to talk to PhD students or similar: How is the work environment? How long do PhD students typically need?
Not PhD specific, but it will come up anyway: "Why did you apply here? Why did you choose this research field, why our group?"