What engineering?

Based on my current experience, PhD is not for industrial practitioners. If you want to study PhD, the academic route should be your ultimate career. PhD explores the knowledge which the industry lacks knowledge on, which also means where the industry is currently not spending money on. Applying whatever you studied in the industry is usually not met with warm reception.

Also, it is generally not recommended to study PhD on a part-time basis. It'll take too long, it will distract you from getting fundings and sponsorships, and your budget will overblow due to inflationary pressure over the years, not to mention it'll be very physically and mentally taxing to do a 8-year (on average) part-time PhD while juggling with a full-time job.

But if you want to do it because you enjoy doing research, then go for it. But I'll still not recommend doing it on a part-time basis.