code: function getContactsByExternalId($externalId, $standard_fields = array(), $custom_fields = array())
      {
          $queryParameters = array(
              'standard_field' => $standard_fields
          );
  
          $queryParameters = $this->appendArrayFields($queryParameters, 'custom_field', $custom_fields);
  
          return $this->get('contacts/externalid/' . utf8_encode($externalId), $queryParameters);
      }
docs: Retrieves all contacts with a given external ID.
  
  @param string $externalId the external ID to search for
  @param string[] $standard_fields
  @param string[] $custom_fields
  @return \em com_maileon_api_MaileonAPIResult
  the result object of the API call, with a com_maileon_api_contacts_Contacts
  available at com_maileon_api_MaileonAPIResult::getResult()
  @throws com_maileon_api_MaileonAPIException
  if there was a connection problem or a server error occurred