In the PECL sources, a certain vendor specific string is included for Microsoft systems.
However, if you just want to pass something entirely different, like Cisco specifics, that's possible.
The fact that this isn't included in the sources of the PECL doesn't mean you can't use them.
Better said, everything there is in the dictionary of your Radius server can be put in as string.
Like the following example to write a h323-gw-id:
<?php
$res = radius_acct_open ();
radius_add_server($res,'127.0.0.1', 1813, 'mysecret', 10, 2);
radius_create_request($res, RADIUS_ACCOUNTING_REQUEST);
radius_put_vendor_string($res,9,33 ,"h323-gw-id=123456");
$result = radius_send_request($res);
?>