It requires changing getQuestionsById(), since getQuestionsById is getting all of the questions and returning a list of questions. This seems like quite a bit of overhead, because then the php needs to get changed too. And it is unlikely that they will have a horribly large amount of favorite questions. I think it is OK, but it is doable if you think it is important.

You are right though, limit and offset would be better for scalability.