if (!isset($_SESSION['forum']) or isset($_SESSION['forum']) && $_SESSION['forum'] != $id)
или
if (!isset($_SESSION['forum']) || (isset($_SESSION['forum']) && $_SESSION['forum'] != $id)) {
if (!isset($_SESSION['forum']) or isset($_SESSION['forum']) && $_SESSION['forum'] != $id)
или
if (!isset($_SESSION['forum']) || (isset($_SESSION['forum']) && $_SESSION['forum'] != $id)) {