When does a PhD student officially stop being a PhD student? When does a PhD student officially stop being a PhD student? To be more specific, are you no longer a PhD student when:


you have submitted your PhD (and thus stopped working on it),
been approved for the degree, or
graduated?


I have looked on the Internet and on Stack Exchange and I cannot find a clear answer to this question. On Academia Stack Exchange, one question asks something similar but from such a different perspective that it provides no help. While I know I can ask some administrative body in the university (and have), I remain sufficiently confused about the general consensus on this matter that I would appreciate answers from experts here.

I have a personal interest in knowing the answer to this question as I am involved in a society which is only open to current students and unsure as to when I will become ineligible.
As I say in the comments and djechlin says in his answer, one's official student status is entirely determined by the institution in which the student is enrolled.  You can ask them at any given time whether you are a student, and they'll tell you.  If you want to know when you will lose your student status assuming that you complete all the requirements and paperwork for the PhD, they should be able to tell you that.  This won't have any cosmic significance though: in practice, you are probably asking them about how the computers in the registrar's office deal with the boundaries between one semester, the next and/or the intervening summer.

In terms of your intended need -- i.e., continued membership in a society -- I suspect that you may be taking the "official student status" more seriously than others will and thereby worrying yourself needlessly.  I can't think of any society that wouldn't take the student's own word as to the completion of their own student status, even allowing a reasonable grace period.  For instance, there are professional societies in my field (mathematics) which students can join for special rates (sometimes free).  In these cases membership is determined annually and the renewal process takes place shortly after a given academic year is underway.  So if you get your PhD in, say, June, then you get to keep the student benefits until, say, September or October.  

A big exception to the above is immigration issues.  For that: yes, ask the university for the precise date at which your student status expires!