function LoadClass()
{
    $db = new DB_Connect();
    $major_id = (string) $_POST['major_id'];
    $statement = 'select * from class where state = 1 and major_id=:major_id';
    $result = $db->prepare($statement);
    $result->execute(array(':major_id' => $major_id));
    $index = 0;
    $ret = "";
    while ($row = $result->fetch()) {
        $element = new classes();
        $element->setVal($row['class_id'], $row['class_name']);
        $ret[$index++] = $element;
    }
    echo json_encode($ret);
}