数据库连接有关问题

文件1
$search_db = new SearchDo();
if ($search_db->connect() === false) { //数据库连接
   exit;
}

if( $area_kubun_id == '') {
        //地域
        $list_area = new AreaList();
        $area_list = $list_area->createList();
        $smarty->assign('area_list', $area_list);
}else {

        $area_list =$search_db->getAreaById($area_kubun_id);
        $smarty->assign('area_list', $area_list);

}
文件2
class AreaList extends SelectList {
    function AreaList(){
        $sql = SQL_AREA_LIST_SELECT;
        $this->sql = $sql;
        $this->SelectList();
    }
}
文件3
class SelectList extends BoatDb {

    function SelectList() {
        $this->BoatDb();
        $this->connect(); // 数据库连接

    }

     function createList() {
        --数据库操作--
        $this->close();// 关闭连接
     }
}