Quick Cookie Question

Hi, Lets say I set a cookie like this: setcookie(session_name(), $szSessionID, time()+$expireTime, "/"); The cookie does get set fine. Now, how can I change the cookie value $szSessionID without affecting the cookies expire time? Will $_COOKIE['szSessionID'] = $newsessionid store the new value to the cookie? So if the cookie time was set to expire in 7 days, I would like to be able to change the cookies value 3 days later, but the expire time would not be reset to 7 days, it would still have the original time which would mean 4 days left to the cookie life. If I can retreive the current expire time of the cookie, I could just subtract that value, but I don't know how to get the cookies set expire time. And I don't want to store the expire time as another cookie or part of this cookie. Thanks for any help. Chris