Set Link header in PHP so it can work in a subfolder

.htaccess

@@ -36,5 +36,4 @@ FileETag None
     Header set X-Content-Type-Options nosniff
     Header set X-XSS-Protection "1; mode=block"
     Header set Referrer-Policy no-referrer
-    Header add Link "</css/style.css>; rel=preload" "expr=%{CONTENT_TYPE} =~ m#text/html#"
 </ifmodule>

classes/LinkHeaderMiddleware.php

@@ -0,0 +1,46 @@
+<?php
+
+namespace Alltube;
+
+use Psr\Container\ContainerInterface;
+use Slim\Http\Request;
+use Slim\Http\Response;
+use Slim\Router;
+
+/**
+ * Class LinkHeaderMiddleware
+ * @package Alltube
+ */
+class LinkHeaderMiddleware
+{
+    /**
+     * @var Router
+     */
+    private $router;
+
+    /**
+     * LinkHeaderMiddleware constructor.
+     * @param ContainerInterface $container
+     */
+    public function __construct(ContainerInterface $container)
+    {
+        $this->router = $container->get('router');
+    }
+
+    /**
+     * @param Request $request
+     * @param Response $response
+     * @param callable $next
+     * @return mixed
+     */
+    public function __invoke(Request $request, Response $response, callable $next)
+    {
+        $response = $response->withHeader(
+            'Link',
+            '<' . $this->router->getBasePath() . '/css/style.css>; rel=preload; as=style'
+        );
+
+
+        return $next($request, $response);
+    }
+}

index.php

@@ -8,6 +8,7 @@ use Alltube\Controller\FrontController;
 use Alltube\Controller\JsonController;
 use Alltube\CspMiddleware;
 use Alltube\ErrorHandler;
+use Alltube\LinkHeaderMiddleware;
 use Alltube\LocaleManagerFactory;
 use Alltube\LocaleMiddleware;
 use Alltube\LoggerFactory;
@@ -44,6 +45,7 @@ try {
     $app->add(new LocaleMiddleware($container));
     $app->add(new RouterPathMiddleware($container));
     $app->add(new CspMiddleware($container));
+    $app->add(new LinkHeaderMiddleware($container));
 
     // Controllers.
     $frontController = new FrontController($container);