If you're a domestic student you can study for a PhD part-time. You'll need to provide the reasons why you need to study part-time when applying for admission, because the University wants to ensure that you'll have enough time to complete your PhD.
It's also possible to change to part-time study during your PhD, but again a reason would need to be given and formal approval sought. You can only change to part-time at the start of a semester.
If you're an international student you need to check with the Student Visa Services team at the International office (email firstname.lastname@example.org) to determine if you're able to study part-time. Typically a student visa does not permit part-time study.