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. See also Session handling.
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!
Note: When using session cookies, specifying an
idfor 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.
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).