In my personal opinion, having been an R&D engineer, working in a semiconductor fab for 28 years, and leading large engineering groups since 1995, you've got to be extraordinarily lucky to be able to work on a PhD while working full time. A master's degree can be done while working because it is largely coursework, and the coursework requirements for a PhD can be similarly done, although both will likely take much longer because there is no way to take multiple courses simultaneously while still doing justice to your employment. However, I don't see any way to do PhD-quality research while being employed full time unless you can somehow leverage the work you do at your job into your thesis. And that is going to be extraordinarily rare. I've seen it happen, but it's not very common. Doing thesis research while being employed full time (at least in engineering) is a pipe dream IMO.