Slim 3 Middleware - Force HTTPS

index.php

$app->add(function (Request $request,  Response $response, $next) {
    if ($request->getUri()->getScheme() !== 'https') {
        $uri = $request->getUri()->withScheme("https")->withPort(null);
        return $response->withRedirect( (string)$uri );
    } else {
        return $next($request, $response);
    }
});

The Request type hint undestands itself under current namespace, if not “use-statemented” otherwise. So you possibly forgot a use statement in your script, like this:

use Psr\Http\Message\ServerRequestInterface as Request;