我一直在獲取要使用mysqli返回的行數方面遇到麻煩。即使確實有一些結果，我每次都會得到0。if($stmt = $mysqli->prepare("SELECT id, title, visible, parent_id FROM content WHERE parent_id = ? ORDER BY page_order ASC;")){      $stmt->bind_param('s', $data->id);      $stmt->execute();    $num_of_rows = $stmt->num_rows;      $stmt->bind_result($child_id, $child_title, $child_visible, $child_parent);      while($stmt->fetch()){        //code    }    echo($num_of_rows);    $stmt->close();}為什么沒有顯示正確的數字？