An ultra-naiive universal middleware host

middleware-host.php

<?php

// an ultra-simple middleware host that works for middleware callables with any signature.
// the last argument is always the `$next` function, which delegates to the next middleware.

class Host
{
    /**
     * @var callable[]
     */
    private $stack;

    /**
     * @param callable[] $stack
     */
    public function __construct(array $stack)
    {
        $this->stack = $stack;
    }

    /**
     * Dispatch the middleware stack
     *
     * @return mixed
     */
    public function __invoke()
    {
        $args = func_get_args();

        return call_user_func_array($this->delegate(0), $args);
    }

    /**
     * @param int $index
     *
     * @return callable
     */
    protected function delegate($index)
    {
        if (isset($this->stack[$index])) {
            return function() use ($index) {
                $args = func_get_args();

                $args[] = $this->delegate($index + 1);

                return call_user_func_array($this->stack[$index], $args);
            };
        }

        return function() {
            throw new RuntimeException("middleware stack exhausted");
        };
    }
}

// example with request only:

$host = new Host([
    function ($request, $next) {
        return $next($request);
    },
    function ($request, $next) {
        return "response({$request})";
    }
]);

echo $host("request"); // => "response(request)"

echo "\n\n";

// example with request and response:

$host = new Host([
    function ($request, $response, $next) {
        return $next($request, $response);
    },
    function ($request, $response, $next) {
        return "handled({$request}, {$response})";
    }
]);

echo $host("request", "response"); // => "handled(request, response)"