code: 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;
      }
docs: Calling this method will establish the hand shake with Twitter OAuth as an application
  and return this sessions Bearer Token to be used for the call being made
  @param  [string] $consumer_key    Your Twitter Application Consumer Key
  @param  [string] $consumer_secret Your Twitter Application Consumer Secret Key
  @return [string]                  Twitter OAuth Bearer Access Token