$sql = "SELECT * from idea ORDER BY datetime DESC LIMIT 50;";

$result = $pdo->query($sql);

if($result->rowCount() > 0 && !empty($result)) // line 6

    {

        foreach ($result as $row)

        {

        $id = $row['id'];

        $title = $row['title'];

        $idea = $row['idea'];

        echo '<span class="idea" id="'.$id.'"><strong style="color: #0081C5">' . $title . "</strong> - " . $idea . ' <a class="delete" href="#">[Delete]</a></span>';

        }

    }
一般如果你的sql语句查询出现错误时，没有进行$result 的判断，直接使用$result -> rowCount()会出现这种错误。