Get an access_token with the Google API Client to authenticate your PHP Backend with google services, like Firebase, Cloud Storage, etc

GoogleApiWrapper.php

public function getServiceAccountAccessToken() {
        
        // put the serviceAccount credentials in the env variable
        putenv('GOOGLE_APPLICATION_CREDENTIALS=../resources/json/serviceAccount.json');

        // Add the scopes (here are the scopes to Firebase Auth)
        $scopes = [
            "https://www.googleapis.com/auth/userinfo.email",
            "https://www.googleapis.com/auth/firebase.database"
        ];

        $client = new Google_Client();
        $client->useApplicationDefaultCredentials();
        $client->setScopes($scopes);

        // Get the token
        $fetchedToken = $client->fetchAccessTokenWithAssertion();

        if(!isset($fetchedToken['access_token'])) {
            throw new \Exception("Error obtaining the access_token", 1);
        }

        return $fetchedToken['access_token'];
            
    }