The session_unset() function frees all session variables
currently registered.
Return Values
No value is returned.
Notes
Note:
If $_SESSION (or $HTTP_SESSION_VARS
for PHP 4.0.6 or less) is used, use unset() to
unregister a session variable, i.e.
unset ($_SESSION['varname']);.
Caution
Do NOT unset the whole $_SESSION with
unset($_SESSION) as this will disable the registering
of session variables through the $_SESSION superglobal.