How should you handle a question in an interview for a PhD program when you don't know the answer ?
Should you try to just say something even if this is wrong or just tell the interviewer that you don't know the answer?
Honesty would be the best and tactical strategy. No one is expected to know everything. An ideal panel would only be interested in how much you know and how much of that pertains to your candidate position as a PhD scholar.
The time given for you in the interview process is gold -- you must try to expose your potential as much as you are able to within the period. If you spend too much time in an a question you cannot make out the answer, then you are probably losing time to express the answers for the future questions you might know. Hence it would be better to admit that you do not know the answer and try if you are confident (as suggested by @StringBad) or move to the next possible question if not.