code: public function generate_mandatory_params() {
  
      // Create new UTC date object
      $date = new DateTime(null, new DateTimeZone('UTC'));
  
      return array(
        'client_id' => $this->account_details['app_id'],
        'nonce'     => GoCardless_Client::generate_nonce(),
        'timestamp' => $date->format('Y-m-d\TH:i:s\Z')
      );
  
    }
docs: Generate mandatory payment parameters: client_id, nonce and timestamp
  
  @return array Mandatory payment parameters