如何使用php pdo在hover上构建bootstrap 4多级下拉导航栏？

<?php
// 连接数据库
$servername = "数据库服务器地址";
$username = "用户名";
$password = "密码";
$dbname = "数据库名";
try {
    $conn = new PDO("mysql:host=$servername;dbname=$dbname", $username, $password);
    // 设置PDO错误模式为异常
    $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
} catch(PDOException $e) {
    echo "数据库连接失败: " . $e->getMessage();
    exit();
}

// 查询顶级导航菜单
$sql = "SELECT * FROM nav_menus WHERE parent_id = 0";
$topNavMenus = $conn->query($sql)->fetchAll(PDO::FETCH_ASSOC);

// 输出顶级导航菜单和对应的子菜单
foreach ($topNavMenus as $topNavMenu) {
    echo '<li class="nav-item dropdown">';
    echo '<a class="nav-link dropdown-toggle" href="#" id="navMenu' . $topNavMenu['id'] . '" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">' . $topNavMenu['name'] . '</a>';
    echo '<div class="dropdown-menu" aria-labelledby="navMenu' . $topNavMenu['id'] . '">';
    // 查询子菜单
    $sql = "SELECT * FROM nav_menus WHERE parent_id = " . $topNavMenu['id'];
    $subNavMenus = $conn->query($sql)->fetchAll(PDO::FETCH_ASSOC);
    // 输出子菜单
    foreach ($subNavMenus as $subNavMenu) {
        echo '<a class="dropdown-item" href="' . $subNavMenu['url'] . '">' . $subNavMenu['name'] . '</a>';
    }
    echo '</div>';
    echo '</li>';
}

// 关闭数据库连接
$conn = null;
?>

CREATE TABLE `nav_menus` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `name` varchar(255) NOT NULL,
  `url` varchar(255) NOT NULL,
  `parent_id` int(11) unsigned NOT NULL DEFAULT '0',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;