Get Google FCM oAuth Token programmatically with PHP

I have a php server that is a backend of an andoid/iphone app. The app registers itself to Firebase, and I would like to send push messages he devices via FCM REST API. It is working well if I get the oAuth Token myself and add hard-coded to the POST message. The goal of this project is to provide me with a PHP code that gets the oAuth Token programmatically, as in: [login to view URL] I expect a working code something like this: // START $client = new Google_Client(); // WHATEVER ..... // Authenticate and get the access token if ($client->isAccessTokenExpired()) { $client->refreshTokenWithAssertion(); } $accessToken = $client->getAccessToken(); // Access token to use with FCM REST API $token = $accessToken['access_token']; echo "Access Token for FCM: " . $token . "\n"; // END You would need to provide me all files needed for it to run (dont tell me to install anything - just send me all files). I have [login to view URL] file if needed.