implements QSA on Symfony2 redirects

QSAListener.php

<?php

use JMS\DiExtraBundle\Annotation as DI;
use Symfony\Component\HttpKernel\Event\FilterResponseEvent;

/** @DI\Service */
class QSAListener
{
    private $blacklist;

    /** @DI\InjectParams({"blacklist"=@DI\Inject("%qsa.blacklist%")}) */
    public function __construct(array $blacklist = array())
    {
        $this->blacklist = $blacklist;
    }

    /** @DI\Observe("kernel.response") */
    public function onKernelResponse(FilterResponseEvent $event)
    {
        $request  = $event->getRequest();
        $response = $event->getResponse();

        if (!$response->isRedirection() || !$request->isMethodSafe() || !$request->query->all()) {
            return;
        }

        $query = array();
        foreach (array_diff($request->query->keys(), $this->blacklist) as $key) {
            $query[$key] = $request->query->get($key);
        }

        // attach the query string
        $location  = $response->headers->get('Location');
        $location .= false === strpos($location, '?') ? '?' : '&';
        $location .= http_build_query($query);

        // modify the response content
        $response->setContent(str_replace(
            htmlspecialchars($response->headers->get('Location'), ENT_QUOTES, 'UTF-8'),
            htmlspecialchars($location, ENT_QUOTES, 'UTF-8'),
            $response->getContent()
        ));

        // modify the location header
        $response->headers->set('Location', $location);
    }
}

I see it is like a secure blocker of IPs, wonder what QSA stands for?

@cordoval in this instance QSA stands for Query String Append. It's an emulation of mod_rewrite's [QSA] flag, which appends any query string parameters in the substitution URL to the query string parameters of the original URL.

Thanks @phuedx!