fcm.php

<?php

// API access token from Google API's Console
define('ACCESS_TOKEN', 'ya29.GlvmBr9UH4KLL_Do6pHGOoZ7ksfURRHAqRh3dPWcMiYjNov4hhse_m147qlSwVZIqty989V73iQrtSD3DVT6AVaNtYFEQbQeq1EuUHhRMBTf1Go6lXTPPMzQX6-Q');
// API access key from Google API's Console
define('API_KEY', 'AIzaSyDB5KH7JaT5zRDq8QyRYOt3xMN4fyBaY_c');
// Firebase project id
define('PROJECT_ID', 'skitto-testbed');

class FCM
{

    function __construct()
    {
        $this->topic_name = 'movies';
    }

    public function subscribe_to_topic()
    {
        $registration_tokens = ['fA3c_wLqvvs:APA91bHpmnkV8vkIPMJ4S6CxN_boB1dXvq2hzlkq0aRQ5FLnznPxpBMGTCpNTfz92sWvUADJ15D0aVPdHJv7ryI7pVLOv-wRMYid9jc1T4F9MrkLM8dZn7J-1ByF3LqqlnNp88iWHz3B'];

        $fields = array(
            "to" => "/topics/" . $this->topic_name,
            "registration_tokens" => $registration_tokens
        );

        $headers = array
        (
            'Content-Type: application/json',
            'Authorization: key=' . API_KEY
        );

        #Send Response To FireBase Server
        $ch = curl_init();
        curl_setopt($ch, CURLOPT_URL, 'https://iid.googleapis.com/iid/v1:batchAdd');
        curl_setopt($ch, CURLOPT_POST, true);
        curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
        curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
        curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($fields));
        $result = curl_exec($ch);
        curl_close($ch);
        echo $result;
    }

    public function send_messages_to_topic()
    {
        $fields = array
        (
            "message" => array(
                "topic" => $this->topic_name,
                "notification" => array(
                    "title" => "Background Message Title",
                    "body" => "Background message body"
                )
            )
        );

        $headers = array
        (
            'Content-Type: application/json',
            'Authorization: bearer '.ACCESS_TOKEN
        );

        $ch = curl_init();
        curl_setopt($ch, CURLOPT_URL, 'https://fcm.googleapis.com//v1/projects/'.PROJECT_ID.'/messages:send');
        curl_setopt($ch, CURLOPT_POST, true);
        curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
        curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
        curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($fields));
        $result = curl_exec($ch);
        curl_close($ch);

        echo $result;
    }
}

$push_notification = new FCM();
$push_notification->subscribe_to_topic();
$push_notification->send_messages_to_topic();