Example StackPHP middleware

gistfile1.php

<?php

use Symfony\Component\HttpKernel\HttpKernelInterface;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;

class EtagMiddleware implements HttpKernelInterface
{
    protected $app;

    public function __construct(HttpKernelInterface $app)
    {
        $this->app = $app;
    }

    public function handle(Request $request, $type = self::MASTER_REQUEST, $catch = true)
    {
        // Run the next middleware (or app)
        $response = $this->app->handle($request, $type, $catch);

        // Calculate the MD5 of the response
        $responseEtag = md5($response->getContent());

        // Validate request's requirements
        if ($request->headers->has('If-None-Match')) {
            if ($request->headers->get('If-None-Match') === $responseEtag) {
                $response->setContent('');
                $response->setStatusCode(304);
            }
        } elseif ($request->headers->has('If-Match')) {
            if ($request->headers->get('If-Match') !== $responseEtag) {
                $e = new PreconditionFailedException();
                $this->terminate($request, exceptionDecorator($e));
                throw $e;

            }
        }

        $response->headers->set('Etag', $responseEtag);

        return $response;
    }
}