PHP编写的语句循环JOIN查询不起作用

I am trying to gather some variables from my query then output them using $stmt->bind_result() however I am not getting any rows returned in my search.

I am not sure what is going wrong here?

  $tag = trim($_GET['tag']);

  $stmt = $mysqli->prepare('SELECT trips.trip_id FROM trips JOIN tags ON trips.post_id = tags.post_id WHERE tag = ?');
  $stmt->bind_param('s', $tag);
  $stmt->execute();
  $stmt->bind_result($id);    

  while ($stmt->fetch()) {

      echo $id;

  }