session_id() returns the session
id for the current session.
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!
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.
See also session_start(),
session_set_save_handler(), and
session.save_handler.