error in php Catchable fatal error: Object of class mysqli_result could not be converted to string in

error in php Catchable fatal error: Object of class mysqli_result could
not be converted to string in

this is query
SELECT room_id,stages FROM booked_room
this is error
Catchable fatal error: Object of class mysqli_result could not be
converted to string in
$query ="SELECT room_id,stages FROM booked_room";
$result2 =$mysqli->query($query);
echo $mysqli->error;
if($result2->num_rows > 0){echo"some thing";}