firebase example.php

<?php
 #class declaration
class Firebase_model extends CI_model
{
  public function __construct()
  {
    parent::__construct();

    define('FIREBASE_API_KEY', 'AAAAh66AGWk:APA91bE83vgJIkt-Edb9iStKIWErZ0iE2UvQR8hXM_iQOzNwZu25C1IChOn8GmxyWg5yzMuCpFa55UhMvoqRLzS0hPZt4jUtQUjMq6IHbMMHMVM3IbguGLgQCqKmneqKIG1c9SWi7khL1g7GkOLiase7xPtvJ_WKIw');
  }

  // sending push message to single user by firebase reg id
  public function send($to, $message, $notification) {
    $fields = array(
      'to' => $to,
      'notification' => $notification,
      'data' => $message,
      'priority' => 'high'
    );
    return $this->sendPushNotification($fields);
  }

  // Sending message to a topic by topic name
  public function sendToTopic($to, $message, $notification) {
    $fields = array(
      'to' => '/topics/' . $to,
      'notification' => $notification,
      'data' => $message,
      'priority' => 'high'
    );
    return $this->sendPushNotification($fields);
  }

  // sending push message to multiple users by firebase registration ids
  public function sendMultiple($registration_ids, $message, $notification) {
    $fields = array(
      'to' => $registration_ids,
      'notification' => $notification,
      'data' => $message,
      'priority' => 'high'
    );
    return $this->sendPushNotification($fields);
  }

  // function makes curl request to firebase servers
  private function sendPushNotification($fields) {
    // Firebase API Key
    // Set POST variables
    $url = 'https://fcm.googleapis.com/fcm/send';
    $headers = array(
      'Authorization: key=' . FIREBASE_API_KEY,
      'Content-Type: application/json'
    );
    // Open connection
    $ch = curl_init();
    // Set the url, number of POST vars, POST data
    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_POST, true);
    curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    // Disabling SSL Certificate support temporarly
    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
    curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($fields));
    // Execute post
    $result = curl_exec($ch);
    if ($result === FALSE) {
      die('Curl failed: ' . curl_error($ch));
    }
    // Close connection
    curl_close($ch);
    return $result;
  }


} # end of class



##########################
#### Sample usage ########
##########################

$firebase_response = $this->firebase_model->sendToTopic($topic_name_string,
       ['data' => []],
       ['title' => 'New notification', 'body' => 'Hello world']
    );