There's an expectation that you will work full-time on your PhD, so if you have a job on top of that, you will be working a lot of hours. Speaking from my own experiences, I have taken on small amounts of university-related work (teaching, invigilating etc) because it's good experience, but I would have found it difficult to have a regular job as well as my PhD. It depends on your circumstances, and I would also say it's important to check the conditions of your PhD, as there may be a specific restriction on how many hours of work you can take on.