Tbf, if it's not a lab-based PhD, you'll usually manage to work on the side. I know several people who were in full-time employment throughout pretty much their whole PhD (aided by supportive employers and sabbaticals). Even at (elite) universities that had explicit rules against it, and cases where the university and the jobs were in different countries. I even know one guy who did a lab-based engineering PhD and continued doing a bit of consulting work for his old employer. It's not even that uncommon among people with prior work experience and skills that fit well with consulting/freelance work. You'll see it on LinkedIn. Internships are also common in many disciplines like Math and CS, and good advisors can even help you get them (I know a prof who sends at least one grad student to intern at Google every year).

It's not advertised and usually not recommended, but it's definitely possible. The key is to find a supervisor who's fine with it, don't even bother asking the administrators.