This advice was given to one of my students (from another student) who was considering whether to leave school and work on a PhD part-time or not ... this is sound advice.
"If you are not too distressed by your current income level, then my counsel is do the PhD full time. The major reason is, to be successful either at work or in the PhD program, you will need total dedication, and you cannot and therefore should not do both at the same time. There are other reasons. A PhD straight from a MS program in a field that you have worked on your MS thesis is much more valuable than any part-time PhD. Most part time PhD programs are never completed for a varieties of reasons, many of which are beyond your control. It is better to do it when you are still young"
I have and still do advise both full-time (Untch, Abdurazik, Li) and part-time (Alexander, Jin, Lee, Hayes, Guan) students. It is possible to be successful in both ways, but the PhD is easier and more rewarding if you can do it full-time.