PDO查询不成功！该如何处理

 error_reporting(0);
 $dsn = "mysql:host=localhost;dbname=liuyan";
 $db = new PDO($dsn, 'root', 'root');
 $name = $_POST['name'];
 $password = md5($_POST['password']);
 //$date = date("Y-m-d H:i:s");
 $sql = "select * from admin where user_name=? and password=?";
 $stm = $db->prepare($sql);
 $stm->execute(array($name,$password));
 $rs=$stm->fetch();
 var_dump($rs);
 if(empty($rs)){
    echo "";
} else{
    echo "";
}
 ?>