public function authenticateApp ($consumer_key, $consumer_secret) {

        $bearer_token = $this->encodeBearer($consumer_key, $consumer_secret);

        // Twitter Required Headers
        $headers = array(
            'Authorization' => 'Basic ' . $bearer_token,
            'Content-Type' => 'application/x-www-form-urlencoded;charset=UTF-8'
        );

        // Twitter Required Body
        $body = 'grant_type=client_credentials';
        
        $response = $this->client->post('/oauth2/token', $headers, $body)->send();
        $data = $response->json();
        $this->bearer = $data['access_token'];
        return $this->bearer;
    }