內容長度中間件會自動附加 Content-Length 標頭至回應。這是為了取代從 Slim 3 中移除的 addContentLengthHeader 設定。此中間件應放置在中間件堆疊的最後，以便先執行，最後退出。
<?php
use Slim\Factory\AppFactory;
use Slim\Middleware\ContentLengthMiddleware;
require __DIR__ . '/../vendor/autoload.php';
$app = AppFactory::create();
// Add any middleware which may modify the response body before adding the ContentLengthMiddleware
$contentLengthMiddleware = new ContentLengthMiddleware();
$app->add($contentLengthMiddleware);
// ...
$app->run();