The Content Length Middleware will automatically append a Content-Length header to the response.
This is to replace the addContentLengthHeader setting that was removed from Slim 3.
This middleware should be placed on the end of the middleware stack so that it gets executed first and exited last.
<?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();