Lumen 5.4 subfolder install

.env

BASE_URL=/subfolder

app_Routing_LumenUrlGenerator.php

<?php

/**
 * Exact copy of vendor/laravel/lumen-framework/src/routing/UrlGenerator.php
 * Renamed to LumenUrlGenerator
 */
 
namespace Laravel\Lumen\Routing;

class LumenUrlGenerator
{
  // lumen-framework/src/routing/UrlGenerator.php
}

app_Routing_UrlGenerator.php

<?php

/**
 * To ease the pain of overriding helpers like url() and route()
 * Little hack with LumenUrlGenerator and composer
 */

namespace Laravel\Lumen\Routing;

class UrlGenerator extends LumenUrlGenerator
{
    protected function getRootUrl($scheme, $root = null)
    {
        if (is_null($this->cachedRoot)) {
            $this->cachedRoot = $this->app->make('request')->root();
            $baseUrl = env('BASE_URL');
            if ($baseUrl && strpos($this->cachedRoot, $baseUrl) === FALSE) {
                $this->cachedRoot .= $baseUrl;
            }
        }
        return parent::getRootUrl($scheme, $root);
    }
}

bootstrap_app.php

// ...

$app->group(['namespace' => 'App\Controllers', 'prefix' => env('BASE_URL', '')], function ($app) {
    require __DIR__.'/../routes/web.php';
});

// ...

composer.json

{
    "autoload": {
        "psr-4": {
            "Laravel\\Lumen\\Routing\\": "app/Routing"
        }
    },
}

TestCase.php

<?php

abstract class TestCase extends Laravel\Lumen\Testing\TestCase
{
    public function setUp()
    {
        parent::setUp();
        
        $this->baseUrl = $this->baseUrl . env('BASE_URL', '');
    }
}

Since I wasn't going to find an easy fix like laravel/lumen-framework#35