gistfile1.php

<?php
use Psr\Http\Message\ServerRequestInterface as Request;
use Psr\Http\Message\ResponseInterface as Response;

class Dispatcher
{
    protected $queue;
    protected $resolver;

    public function __construct(array $queue, callable $resolver)
    {
        $this->queue = $queue;
        $this->resolver = $resolver;
    }

    public function __invoke(Request $request, Response $response)
    {
        $spec = array_shift($queue);
        if (! $spec) {
            $spec = function ($request, $response) { return $response; };
        }

        $middleware = call_user_func($this->resolver, $spec);
        return $middleware($request, $response, $this);
    }
}