Is it common to cite conference papers in a PhD? I tried searching for a similar question but most I read dealt with published papers and not a PhD, so apologies if this is a duplicate. I will remove this question if it is.

I've read a number of PhD dissertations and I haven't come across one which cites a conference paper, only journals (and the occasional website). I guess also it depends on the university if they accept such citations but I was just wondering if this is commonplace?
It has little to do with paper vs. thesis and everything to do with whether the person's field considers conference papers "real" publications.  If you read computer science Ph.D. dissertations, you will find them to be filled with conference paper citations.  In a field that disdains conferences, you will not.