Unreasonable Errors on PHP Slim 3 Middleware

Question

I am trying to use the ValidationErrorsMiddleware.php class as a middleware, so I added the following code to my bootstrap/app.php:

$app->add(new App\Middleware\ValidationErrorsMiddleware($container));

I got the following errors after the above code is added to my app.php:

Fatal error: Uncaught exception 'RuntimeException' with message 'Unexpected data in output buffer. Maybe you have characters before an opening <?php tag?' in C:\wamp64\www\authentication\vendor\slim\slim\Slim\App.php on line 552
RuntimeException: Unexpected data in output buffer. Maybe you have characters before an opening <?php tag? in C:\wamp64\www\authentication\vendor\slim\slim\Slim\App.php on line 552

Just in case, anyone needs to look at the code of my classes and app.php, I have included them down here

---

ValidationErrorsMiddleware.php

<?php

namespace App\Middleware;

class ValidationErrorsMiddleware extends Middleware {

  public function __invoke($request, $response, $next) {

    var_dump('middleware');
    $response = $next($request, $response);

    return $response;
  }
}

Middleware.php

<?php

namespace App\Middleware;

class Middleware {

protected $container;

  public function __construct($container) {

    $this->container = $container;
  }
}

App.php

<?php

session_start();

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

$app = new \Slim\App([
'settings' => [
    'determineRouteBeforeAppMiddleware' => false,
    'displayErrorDetails' => true,
    'db' => [
        // Eloquent configuration
        'driver' => 'mysql',
        'host' => 'localhost',
        'database' => 'phpdb',
        'username' => 'root',
        'password' => 'root',
        'charset' => 'utf8',
        'collation' => 'utf8_unicode_ci',
        'prefix' => '',
    ]
],
]);


$container = $app->getContainer();

$app->add(new App\Middleware\ValidationErrorsMiddleware($container));

require __DIR__ . '/../app/routes.php';

Answer 1

I've fixed my problem doing this:

return [
'settings' => [
    // Slim Settings
    'determineRouteBeforeAppMiddleware' => true,
    'displayErrorDetails' => true,
    'addContentLengthHeader' => false,

I added the attribute addContentLengthHeader with the value false in the settings array.

But I still did not understand what this is for

UPDATE

This problem happens because of the line var_dump(middleware) thats changes the content length of the response. My solution was just a hack. Thanks to @iKlsR for the right answer.