I am working on a project which was created with Symfony 2.8 and have some problems when trying to migrate to the FCM HTTP V1 API.

What is the procedure? I would be very grateful for your help in solving this problem.

Here are the relevant code snippets for the current FCM implementation:

 $tokens,
            'data' => $message

        );
        $headers = array(
            'Authorization:key = ' . $key,
            'Content-Type: application/json'
        );
        $ch = curl_init();
        curl_setopt($ch, CURLOPT_URL, $url);
        curl_setopt($ch, CURLOPT_POST, true);
        curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
        curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
        curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
        curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($fields));
        $result = curl_exec($ch);
        if ($result === FALSE) {
            die('Curl failed: ' . curl_error($ch));
        }
        curl_close($ch);
        return $result;
    }

    function send_notification($tokens, $message, $key)
    {
        $url="https://fcm.googleapis.com/fcm/send";
        $fields = array(
            'to' => '/topics/Flixo',
            'data' => $message
        );
        $headers = array(
            'Authorization:key = ' . $key,
            'Content-Type: application/json'
        );
        $ch = curl_init();
        curl_setopt($ch, CURLOPT_URL, $url);
        curl_setopt($ch, CURLOPT_POST, true);
        curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
        curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
        curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
        curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($fields));
        $result = curl_exec($ch);
        if ($result === FALSE) {
            die('Curl failed: ' . curl_error($ch));
        }
        curl_close($ch);
        return $result;
    }

    public function privacypolicyAction()
    {
        $em = $this->getDoctrine()->getManager();
        $setting = $em->getRepository("AppBundle:Settings")->findOneBy(array(), array());
        return $this->render("AppBundle:Home:privacypolicy.html.twig", array("setting" => $setting));
    }

    public function apprefundpolicyAction()
    {
        $em = $this->getDoctrine()->getManager();
        $setting = $em->getRepository("AppBundle:Settings")->findOneBy(array(), array());
        return $this->render("AppBundle:Home:apprefundpolicy.html.twig", array("setting" => $setting));
    }

    public function notifChannelAction(Request $request)
    {
        $imagineCacheManager = $this->get('liip_imagine.cache.manager');
        $em = $this->getDoctrine()->getManager();
        $defaultData = array();
        $form = $this->createFormBuilder($defaultData)
            ->setMethod('GET')
            ->add('title', TextType::class)
            ->add('message', TextareaType::class)
            ->add('object', 'entity', array('class' => 'AppBundle:Channel'))
            ->add('icon', UrlType::class, array("label" => "Large Icon", "required" => false))
            ->add('image', UrlType::class, array("label" => "Big Picture", "required" => false))
            ->add('send', SubmitType::class, array("label" => "Send notification"))
            ->getForm();
        $form->handleRequest($request);
        if ($form->isSubmitted() && $form->isValid()) {
            $data = $form->getData();
            $selected_channel = $em->getRepository("AppBundle:Channel")->find($data["object"]);
            $message = array(
                "type" => "channel",
                "id" => $selected_channel->getId(),
                "title" => $data["title"],
                "message" => $data["message"],
                "image" => $data["image"],
                "icon" => $data["icon"]
            );

            $setting = $em->getRepository('AppBundle:Settings')->findOneBy(array());
            $key = $setting->getFirebasekey();
            $message_image = $this->send_notification(null, $message, $key);
            $this->addFlash('success', 'Operation has been done successfully ');
        }
        return $this->render('AppBundle:Home:notif_channel.html.twig', array(
            "form" => $form->createView()
        ));
    }
}