code: public function create_bill($params) {
  
      if ( ! isset($params['pre_authorization_id'])) {
        throw new GoCardless_ArgumentsException('pre_authorization_id missing');
      }
  
      $pre_auth = new GoCardless_PreAuthorization($this, array(
        'id' => $params['pre_authorization_id']
      ));
  
      return $pre_auth->create_bill(array('amount' => $params['amount']));
  
    }
docs: Create a new bill under a given pre-authorization
  
  @param array $params Must include pre_authorization_id and amount
  
  @return string The new bill object