session_set_cookie_params

i have a problem with the session cookie expiration. i'm writing a login module based upon a session that is started on every page request to store the data of the user that is currently logged in. now i want a "remember me" checkbox that makes the module set the expiration time to a certain value. the problem is that if this expiration time > 0 is not set on the FIRST page request after opening the browser (i.e. the request that would only display the login form), it is already set to zero and the set_cookie_params function does not alter the expiration time afterwards. how can i reset the expiration time of a session if the browser has already a cookie (with lifetime zero) in memory? tia, sam