$client->lists->getListMembersInfo($list_id='xxxxx', $fields='members.email_address,members.status,members.full_name'], null, 20);

specify other parameters in the right order. Third parameter is excluded_fields, fourth is count. Refer to 
https://stackoverflow.com/questions/66500584/adding-query-parameters-to-mailchimp-api-request-for-php