code: public function getBearerToken()
      {
          $url = $this->getBearerTokenUrl();
  
          $params = array(
              'post' => array('grant_type' => 'client_credentials'),
              'headers' => $this->buildBearerTokenHeader(),
          );
  
          $response = $this->curl->send($url, $params);
  
          $obj = json_decode($response['body']);
  
          if (!$obj || !isset($obj->token_type) || $obj->token_type != 'bearer') {
              $this->findExceptions($response);
          }
  
          $this->bearerToken = rawurldecode($obj->access_token);
  
          unset($url, $params, $response, $obj);
  
          return $this->bearerToken;
      }
docs: Get a bearer token from consumer keys
  
  @return null|string  Bearer token
  @throws \TwitterOAuth\Exception\CurlException
  @throws \TwitterOAuth\Exception\TwitterException