session_id() is used to get or set the session id for the current session.
The constant SID can also be used to retrieve the current name and session id as a string suitable for adding to URLs. Note that SID is only defined if the client didn't send the right cookie. See also Session handling.
If id is specified, it will replace the current session id. session_id() needs to be called before session_start() for that purpose. Depending on the session handler, not all characters are allowed within the session id. For example, the file session handler only allows characters in the range a-z, A-Z and 0-9!
注: 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.