I just realized (although I should have known when I was using $_SESSION :-)) that you cannot perform any search or filtering if you have cookies disabled in your browser. You alway get the full list of publications.
But it is possible to do every search and filtering using the URL. So I wonder if it wouldn't be better to drop the use of the $_SESSION variable completely and just redirect to the matching URL if search or filters are selected. Then everything works without cookies and users don't complain "The filters are broken...".
What do you think about this?