FCM utility for PHP.

Auth_Util.php

<?php
defined('BASEPATH') or exit('No direct script access allowed');

class Auth_Util {
    private $firebaseConfig;
    private $scope;

    public function __construct() {
        $this->CI =& get_instance();
        $this->CI->load->library('FirebaseServiceConfig');
        $this->firebaseConfig = new FirebaseServiceConfig();
        //can be changed to another scope too
        // refer this for more info: https://developers.google.com/identity/protocols/oauth2/scopes
        $this->scope = "https://www.googleapis.com/auth/firebase.messaging";  
    }

    public function generateAccessToken() {
        $jwtToken = $this->createJwtToken();
        $data = array(
            'grant_type' => 'urn:ietf:params:oauth:grant-type:jwt-bearer',
            'assertion' => $jwtToken
        );

        $ch = curl_init('https://www.googleapis.com/oauth2/v4/token');
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
        curl_setopt($ch, CURLOPT_POST, true);
        curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query($data));

        $response = curl_exec($ch);
        curl_close($ch);
        $jsonResponse = json_decode($response, true);
        return $jsonResponse['access_token'];
    }

    private function createJwtToken() {
        $validForSec = 3600;
        $privateKey = $this->firebaseConfig->private_key;
        $saEmail = $this->firebaseConfig->client_email;

        $header = json_encode(array("alg" => "RS256", "typ" => "JWT"));
        $claim = json_encode(array(
            "iss" => $saEmail,
            "scope" => $this->scope,
            "aud" => "https://www.googleapis.com/oauth2/v4/token",
            "exp" => (time() + $validForSec),
            "iat" => time()
        ));

        $requestBody = base64_encode($header) . '.' . base64_encode($claim);
        openssl_sign($requestBody, $signature, $privateKey, OPENSSL_ALGO_SHA256);
        $signature = base64_encode($signature);

        return $requestBody . '.' . $signature;
    }
}

FirebaseServiceConfig.php

<?php
defined('BASEPATH') or exit('No direct script access allowed');

class FirebaseServiceConfig {
    public $type;
    public $project_id;
    public $private_key_id;
    public $private_key;
    public $client_email;
    public $client_id;
    public $auth_uri;
    public $token_uri;
    public $auth_provider_x509_cert_url;
    public $client_x509_cert_url;
    public $universe_domain;

    //replace those with the real one from your .json file 
    public function __construct() {
        $this->type = "service_account";
        $this->project_id = "project_id";
        $this->private_key_id = "blahblah";
        $this->private_key = "-----BEGIN PRIVATE KEY-----\nblahblah\n-----END PRIVATE KEY-----\n"; 
        $this->client_email = "";
        $this->client_id = "";
        $this->auth_uri = "";
        $this->token_uri = "";
        $this->auth_provider_x509_cert_url = "";
        $this->client_x509_cert_uprivate = "";
        $this->universe_domain = "googleapis.com";
    }
}

Some_Helper_class.php

<?php

class Some_Helper_class {
      private function sendNotification($data, $headers)
    {
        $curl = curl_init();

        //don't forget to change to the correct project-id
        curl_setopt_array($curl, array(
            CURLOPT_URL => "https://fcm.googleapis.com/v1/projects/project-id/messages:send",
            CURLOPT_RETURNTRANSFER => true,
            CURLOPT_ENCODING => "",
            CURLOPT_MAXREDIRS => 10,
            CURLOPT_TIMEOUT => 30,
            CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
            CURLOPT_CUSTOMREQUEST => "POST",
            CURLOPT_POSTFIELDS => json_encode($data),
            CURLOPT_HTTPHEADER => $headers,
        ));

        $response = curl_exec($curl);
        $err = curl_error($curl);

        curl_close($curl);

        log_message('info', 'response: ' . $response);

        if ($err) {
            return "cURL Error #:" . $err;
        } else {
            return $response;
        }
    }
}