php5 连接monggodb数据库

$conn = new MongoDB\Driver\Manager("mongodb://localhost:27017");
问题：
<b>Catchable fatal error</b>: Object of class MongoDB\Driver\Manager could not be converted to string in <b>D:\phpstudy_pro\WWW\web\check.api\controller\MongoController.php</b> on line <b>79</b>