如何限制“；SELECT”；后果

我有了下一个代码，他向用户展示了正在制作的新类别。问题是我不知道如何限制结果，并对最后5个类别进行类似的限制？谢谢抱歉英语不好。感谢

   <?php
    $stmt = $pdo->prepare('SELECT * FROM categories');
    $stmt->execute();
    echo $stmt->rowCount();
?>
</span>
</a>
<ul class="dropdown-menu extended notification">
    <li>
        <p>Categories</p>
    </li>
<?php
    if($stmt->rowCount() > 0) {
        foreach($stmt->fetchAll() as $row) {
            $html = '<li>';
            $html .= '<div class="alert alert-info clearfix">';
            $html .= '<span class="alert-icon"><i class="fa fa-flag"></i></span>';
            $html .= '<div class="noti-info">';
            $html .= 'Category #'.$row['CategoryID'].' '.$row['CategoryName'].'';
            $html .= '</div>';
            $html .= '</div>';
            $html .= '</li>';
            echo $html;
        }
    } else {
        $html = '<li>';
        $html .= '<div class="alert alert-danger clearfix">';
        $html .= '<span class="alert-icon"><i class="fa fa-flag"></i></span>';
        $html .= '<div class="noti-info">';
        $html .= '<a href="#"> There are no available categories.</a>';
        $html .= '</div>';
        $html .= '</div>';
        $html .= '</li>';
        echo $html;
    }
?>