When vendor specific string is not included, you can define it and use in your code:
<?php
define('RADIUS_VENDOR_CISCO', 9);
define('RADIUS_PROXY_STATE', 33);
radius_put_vendor_string($res, RADIUS_VENDOR_CISCO, RADIUS_PROXY_STATE, "h323-gw-id=123456");
?>
List of RADIUS Attribute Types:
https://www.iana.org/assignments/radius-types/radius-types.xhtml#radius-types-2
List of Vendor ID's:
https://www.iana.org/assignments/enterprise-numbers/enterprise-numbers