The session_unset() function frees all session variables
currently registered.
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.
