for sending through google you must use authentication method. which means that you must acquire key.json first from firebase console

1. goto https://console.firebase.google.com/
2. goto the intended project
3. click on cogs button
4. click on service accounts
5. click on Generate New Private Key button.

it should be contain like this

{
  "type": "",
  "project_id": "",
  "private_key_id": "",
  "private_key": "",
  "client_email": "",
  "client_id": "",
  "auth_uri": "",
  "token_uri": "",
  "auth_provider_x509_cert_url": "",
  "client_x509_cert_url": "",
  "universe_domain": "",
}

then you should use require google library.
im using this

composer require google/apiclient

require that on your project like this

require APPPATH . '/libraries/php-google/autoload.php';
use Google\Auth\Credentials\ServiceAccountCredentials;

on my example, i create a function to create a authentication

function getAccessToken() {
    $key = json_decode(file_get_contents('/config/key.json'), true);
    
    $scopes = ['https://www.googleapis.com/auth/cloud-platform']; // Define the scopes you need
    $credentials = new ServiceAccountCredentials($scopes, [
        'client_email' => $key['client_email'],
        'private_key' => $key['private_key']
    ]);
    
    return $credentials->fetchAuthToken()['access_token'];
}

change ‘/config/key.json’ to the path of your key.. best to dont put it on public folder.

here is for send notification function

 0, "message" => "Notification couldn't be sent"];

        $tokenArr = $token;
        $msgNotification = [
            "title" => $title,
            "body" => $body,
        ];
        $extraNotificationData = [
            "notification_id" => "",
            "title" => $title,
            "link" => $link
        ];

        $fcmNotification = [
            "message" => [
                "token" => $tokenArr,
                "notification" => $msgNotification,
                "android" => [
                    "priority" => "normal"
                ],
                "data" => $extraNotificationData
            ]
        ];

        $headers = [
            "Authorization: Bearer ".getAccessToken(),
            "Content-Type: application/json"
        ];

        if ($token == null || $token == '') {
            $result="";
        }
        else {
            $ch = curl_init();
            curl_setopt($ch, CURLOPT_URL,$fcmUrl);
            curl_setopt($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, $encodedData);
            $result = curl_exec($ch);
            if ($result === FALSE) {
                $result = curl_error($ch);
            }
            curl_close($ch);
        }

        $response = ["status" => 1, "message" => "Notification sent to users", "payload" => $result];

        return [$response,$token];
    }
?>