code: public function authorize_url($options = null) {
  
      if ( ! isset($options['redirect_uri'])) {
        throw new GoCardless_ArgumentsException('redirect_uri required');
      }
  
      $required_options = array(
        "client_id" => $this->account_details['app_id'],
        "scope" => "manage_merchant",
        "response_type" => "code"
      );
  
      $params = array_merge($required_options, $options);
      $request = GoCardless_Utils::generate_query_string($params);
  
      return $this->base_url . "/oauth/authorize/?" . $request;
  
    }
docs: Generate the OAuth authorize url
  
  @param array $options The parameters to use
  
  @return string The generated url