$pdo->exec($sql);为什么总是返回false

$pdo = new PDO($dsn,$username,$password);

var_dump($pdo);//输出为object(PDO)#1 (0) { } 

//执行插入语句 

$sql = "update text set age=age+1 where id=5";

$records = $pdo->exec($sql);

var_dump($records);