Subscribe một token tới một topic, có hỗ trợ sub theo mảng token - xem thêm https://developers.google.com/instance-id/reference/server#create_a_relation_mapping_for_an_app_instance

fcm.topic-subscribe.tugo-web.demo.php

<?php

$curl = curl_init();

curl_setopt_array($curl, array(
  CURLOPT_URL => "https://iid.googleapis.com/iid/v1/DEVICE_TOKEN/rel/topics/TOPIC_NAME",
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_ENCODING => "",
  CURLOPT_MAXREDIRS => 10,
  CURLOPT_TIMEOUT => 0,
  CURLOPT_FOLLOWLOCATION => true,
  CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
  CURLOPT_CUSTOMREQUEST => "POST",
  CURLOPT_HTTPHEADER => array(
    "Authorization: key=SERVER_KEY",
    "Content-Type: application/json"
  ),
));

$response = curl_exec($curl);

curl_close($curl);
echo $response;