middleware.php

<?php

use Psr\Http\Message\ResponseRequestInterface;
use Psr\Http\Message\ServerRequestInterface;

function (ServerRequestInterface $request, ResponseInterface $response, callable $next)
{
    // Do something with the Request or stop execution and return a Response here.
    ...

    // Call the next middleware
    $response = $next($request, $response);

    // Modify the response, if you like, before returning it.
    ...

    return $response;
}