Elgg\Http\ResponseFactory::respond PHP Method

    public function respond(ResponseBuilder $response)
    {
        $response_type = $this->parseContext();
        $response = $this->hooks->trigger('response', $response_type, $response, $response);
        if (!$response instanceof ResponseBuilder) {
            throw new InvalidParameterException("Handlers for 'response','{$response_type}' plugin hook must " . "return an instanceof " . ResponseBuilder::class);
        }
        if ($response->isNotModified()) {
            return $this->send($this->prepareResponse('', ELGG_HTTP_NOT_MODIFIED));
        }
        $is_xhr = $this->request->isXmlHttpRequest();
        $is_action = false;
        if (0 === strpos($response_type, 'action:')) {
            $is_action = true;
        }
        if ($is_action && $response->getForwardURL() === null) {
            // actions must always set a redirect url
            $response->setForwardURL(REFERRER);
        }
        if ($response->getForwardURL() === REFERRER) {
            $response->setForwardURL($this->request->headers->get('Referer'));
        }
        if ($response->getForwardURL() !== null && !$is_xhr) {
            // non-xhr requests should issue a forward if redirect url is set
            // unless it's an error, in which case we serve an error page
            if ($this->isAction() || !$response->isClientError() && !$response->isServerError()) {
                $response->setStatusCode(ELGG_HTTP_FOUND);
            }
        }
        if ($is_xhr && $is_action && !$this->ajax->isAjax2Request()) {
            // xhr actions using legacy ajax API should return 200 with wrapped data
            $response->setStatusCode(ELGG_HTTP_OK);
            $response->setContent($this->wrapLegacyAjaxResponse($response->getContent(), $response->getForwardURL()));
        }
        if ($is_xhr && $is_action) {
            // Actions always respond with JSON on xhr calls
            $headers = $response->getHeaders();
            $headers['Content-Type'] = 'application/json; charset=UTF-8';
            $response->setHeaders($headers);
        }
        $content = $this->stringify($response->getContent());
        $status_code = $response->getStatusCode();
        $headers = $response->getHeaders();
        if ($response->isRedirection()) {
            $redirect_url = $response->getForwardURL();
            return $this->redirect($redirect_url, $status_code);
        }
        if ($this->ajax->isReady() && $response->isSuccessful()) {
            return $this->respondFromContent($content, $status_code, $headers);
        }
        if ($response->isClientError() || $response->isServerError() || $response instanceof ErrorResponse) {
            return $this->respondWithError($content, $status_code, $headers);
        }
        return $this->respondFromContent($content, $status_code, $headers);
    }