PHP的PDO的问题

<?phpabstract class AbstractDB {    private $m_pdo;    public  $m_stmt;    private static $empty_array = array();    public function open(){        try {            $this->m_pdo = new PDO('mysql:host=127.0.0.1;dbname=MYDB', 'root', '123456', array(PDO::MYSQL_ATTR_INIT_COMMAND => "SET NAMES 'utf8';"));            return true;        }        catch (PDOException $e){            return false;        }    }    public function close(){        $this->m_stmt = null;        $this->m_pdo = null;    }    public function prepare_statement($prepare_statement_text){        if (empty($this->m_pdo))            return;        if(!empty($this->m_stmt)){            $this->m_stmt->closeCursor();        }        $this->m_stmt = $this->m_pdo->prepare($prepare_statement_text);    }    public function set_param($param_name, $param_value){        if (empty($this->m_stmt))            return;        $this->m_stmt->bindParam($param_name, $param_value);    }    public function execute_single_row(){        if (empty($this->m_stmt))            return self::$empty_array;        if (!$this->m_stmt->execute()){            return self::$empty_array;        }        return  $this->m_stmt->fetch(PDO::FETCH_ASSOC);    }    public function execute_rows(){        if (empty($this->m_stmt))            return self::$empty_array;        if (!$this->m_stmt->execute())        {            $error = $this->m_stmt->errorInfo();            return self::$empty_array;        }        $this->m_stmt->setFetchMode(PDO::FETCH_ASSOC);        return $this->m_stmt->fetchALL();    }}class TCategoryAttrDB extends AbstractDB{    public function  get_category_attr_list($arr_params){        $sql = 'select category_id,category_name,attrib_name,attrib_content from cat_attrib where category_id = :category_id and  is_delete = 0';        $this->prepare_statement($sql);        $this->set_param(':category_id', $arr_params['category_id']);        return $this->execute_rows();    }    public function getCategoryIdFromTCategory($arr_params) {        $sql = 'SELECT category_id FROM `t_category` where category_name = :category_name and is_delete = 0            and parent_id = (SELECT category_id from t_category WHERE category_name = :parent_category_name and is_delete = 0);';        $this->prepare_statement($sql);        $this->set_param(':parent_category_name', $arr_params['parent_category_name']);        $this->set_param(':category_name', $arr_params['category_name']);        return $this->execute_single_row();    }}$t_db = new TCategoryAttrDB();$arr_params = array('parent_category_name' => 'XXXX', 'category_name' => 'YYY');if($t_db->open()){    $category = $t_db->getCategoryIdFromTCategory($arr_params);    $result = $t_db->get_category_attr_list($category);}?>

//(1)在PDO初始化时候设置MYSQL_ATTR_USE_BUFFERED_QUERY属性,PDO::MYSQL_ATTR_USE_BUFFERED_QUERY=>1//(2)在prepare_statement函数中添加下面代码,就是在prepare前执行closeCursor()if(!empty($this->m_stmt)){    $this->m_stmt->closeCursor();}