storing url-vars to session

[Anon]

Hi.
I'm trying to save a variable I get through the URL to a session var (having the screensize saved and not changed when clicking on sections of the page).
Can't get it to work though.

I read somewhere that you couldn't alter session-variables through the URL, but I'm just trying to set it for every user that comes and have it stay the same value throughout their visit.

thanks.