session_id() is used to get or set the session id for the current session.
SID can also be used to
retrieve the current name and session id as a string suitable for
adding to URLs. See also 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
a-z, A-Z and 0-9!
session_id() returns the session id for the current
session or the empty string (
"") if there is no current
session (no current session id exists).