code: public function fetch_access_token($params) {
  
      if ( ! isset($params['redirect_uri'])) {
        throw new GoCardless_ArgumentsException('redirect_uri required');
      }
  
      $params['http_authorization'] = $this->account_details['app_id'] . ':' .
        $this->account_details['app_secret'];
  
      $response = $this->request('post', '/oauth/access_token', $params);
  
      $merchant = explode(':', $response['scope']);
      $merchant_id = isset($merchant[1]) ? $merchant[1] : null;
      $access_token = $response['access_token'];
  
      return array(
        'merchant_id'   => $merchant_id,
        'access_token'  => $access_token
      );
  
    }
docs: Fetch an access token for the current user
  
  @param array $params The parameters to use
  
  @return array Array containing the Merchant ID ('merchant_id') and Access Token ('access_token')