Machine learning PhD Interview technical questions
In general, linear algebra and basic concepts in statistics (probability distributions, marginalization) and machine learning (you should be familiar with terns like Maximum-A-Posteriori estimation and Maximum-Likelihood-Estimation, maybe Markov chains and SVMs, neural-networks, etc.).

Best practice would be to find out who is interviewing you and than look at what courses he is leading and what papers he has published in recent years. He is most likely to ask specific question from his field of expertise and interests.