I find the Note in PHP document. http://www.php.net/manual/en/function.session-id.php
"Note: When using session cookies, specifying an id for session_id() will always send a new cookie when session_start() is called, regardless if the current session id is identical to the one being set." I feel puzzled about this feature. Even if the current session id is identical to the one one being set, session_start will send a new cookie. I want to know why session_start behave in this way. Forgive my poor English. Thanks in advance.