求教关于PDO使用的有关问题

<br />CREATE TABLE `session` (<br />  `skey` char(32) CHARACTER SET ascii NOT NULL,<br />  `data` text COLLATE utf8mb4_bin,<br />  `expire` int(11) NOT NULL,<br />  PRIMARY KEY (`skey`)<br />) ENGINE=MyISAM DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;<br />

<br />define('DNS', 'mysql:host=localhost;dbname=db;charset=utf8mb4');<br />define('USR', 'usr');<br />define('PWD', 'pwd');<br />define('MAXLIFETIME', 1440);<br /><br />function write($id, $data) {<br />	try {<br />		$dbh = new PDO(DNS, USR, PWD, array(<br />			PDO::ATTR_PERSISTENT => TRUE,<br />			PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,<br />			PDO::ATTR_EMULATE_PREPARES => FALSE<br />		));<br />		try {<br />			$expire = time() + MAXLIFETIME;<br />			$sql = 'INSERT INTO `session` (`skey`, `data`, `expire`) '<br />					. 'VALUES (:skey, :data, :expire) '<br />					. 'ON DUPLICATE KEY UPDATE '<br />					. '`data` = :data, `expire` = :expire';<br />			$stmt = $dbh->prepare($sql);<br />			$stmt->bindValue(':skey', $id, PDO::PARAM_STR);<br />			$stmt->bindValue(':data', $data, PDO::PARAM_STR);<br />			$stmt->bindValue(':expire', $expire, PDO::PARAM_INT);<br />			$stmt->execute();<br />			$dbh = NULL;<br />		} catch (Exception $e) {<br />			echo $e->getTraceAsString();<br />		}<br />	} catch (Exception $e) {<br />		echo $e->getTraceAsString();<br />	}<br />}<br /><br />write('12345678', '87654321');<br />

<br />PDO::ATTR_PERSISTENT => TRUE,<br />PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,<br />