需要实现的效果：

http://xx.com/u/RkdJ80      =>     http://xx.com/home/url/url/index

实现步骤

1. 设置tp6隐藏网址的index.php

具体参考官方文档：https://www.kancloud.cn/manual/thinkphp6_0/1037488

2. 然后 config\app.php 设置，开启路由，设置默认路由

    'with_route'       => true,
    'default_app'      => 'home',

3. 写好请求方法，确定正常能访问到

子目录模式：app\home\controller\url\UrlController.php文件下，index方法，确定完整地址能访问：http://xx.com/home/url/url/index

4. 设置应用下路由

设置home应用下的路由定义app\home\route\route.php（目录名固定，文件名随意）下：

<?php

use think\facade\Route;

Route::get('/', 'index.Index/index');

// TODO 短网址实现
// http://demo.demo/u/Udos81d

Route::get('u/:hash',       'url.url/index');
Route::get('u',       'url.url/index');


// controller 二级目录，动态路由

Route::get(':dir/:class/:fun', ':dir.:class/:fun');

Route::post(':dir/:class/:fun', ':dir.:class/:fun');

根据实际情况修改:hash，:hash为方法中param取值的key。

5. 修改入口文件

原入口文件：public/index.php，内容为

<?php
// +----------------------------------------------------------------------
// | ThinkPHP [ WE CAN DO IT JUST THINK ]
// +----------------------------------------------------------------------
// | Copyright (c) 2006-2019 http://thinkphp.cn All rights reserved.
// +----------------------------------------------------------------------
// | Licensed ( http://www.apache.org/licenses/LICENSE-2.0 )
// +----------------------------------------------------------------------
// | Author: liu21st <liu21st@gmail.com>
// +----------------------------------------------------------------------

// [ 应用入口文件 ]
namespace think;

require __DIR__ . '/../vendor/autoload.php';

// 执行HTTP应用并响应
$http = (new App())->http;

$response = $http->run();

$response->send();

$http->end($response);

修改为：

<?php
// +----------------------------------------------------------------------
// | ThinkPHP [ WE CAN DO IT JUST THINK ]
// +----------------------------------------------------------------------
// | Copyright (c) 2006-2019 http://thinkphp.cn All rights reserved.
// +----------------------------------------------------------------------
// | Licensed ( http://www.apache.org/licenses/LICENSE-2.0 )
// +----------------------------------------------------------------------
// | Author: liu21st <liu21st@gmail.com>
// +----------------------------------------------------------------------

// [ 应用入口文件 ]
namespace think;

require __DIR__ . '/../vendor/autoload.php';

// 执行HTTP应用并响应
$http = (new App())->http;


// 短网址修改by OneNine
$uri = trim($_SERVER['REQUEST_URI'], '/');

$uri_arr = explode('/', $uri);

$default_app = "home";


// TODO 新加入应用,需要在这里再次标识
$other_app = ['adm', 'payment'];

if (in_array($uri_arr[0], $other_app)) {
    $response = $http->run();
} else {
    $response = $http->name($default_app)->run();
}

$response->send();

$http->end($response);

注意：
根据实际情况修改以上：$default_app和$other_app里面的值，最终效果如下：