I am on a three year PhD programme which is fully funded. My salary is fair, nothing flashy but definitely enough to get by without having to do anything else.
Recently, I was offered to start a new course and teach it at a different university from the one where I am employed, in exchange for an extra salary. I am very inclined to say yes to this because it is a really good opportunity to gain teaching experience, promote my research center and earn more money.
From a bit of Googling around it seems that there are different takes on this depending on the country, but that the general approach is that bijobs during the PhD is totally fine. My university (a Scandinavian one), however, officially states that applicants to PhD programs cannot have other jobs. It's confusing to me and I'm not comfortable just naively considering it consultancy work rather than an actual job, and playing stupid if bureaucracy catches up.
My supervisor has a very "long leash" advising style so his opinion on the matter is that I should do whatever I'm comfortable with as long as I'm fulfilling my academic duties. I'm pretty comfortable with it, but the rules are fuzzy and what I really want is some examples of what other people have done in this situation.