HTTP_Session and the function setExpire()

Hello everybody, This is the first message i post in this mailing list. I try to use HTTP_SESSION just by read the code and the comments. I'm actually interested by the function setExpire(). but this function doesn't seems to have any effect on the value of the expiration date of the cookie. So, do i have to write my own code to update the cookie to keep it for future visits ? thanks