RADIUS Access-Challenge packets (PHP)

Hi.

I have been using RADIUS Authentication php scripts to do an 
implementation of SecurID. So far it has been working well but I have 
hit a bit of a snag.

I have the script working fine for Accept and Reject status, but I am 
having trouble with handling a challenge response. The response comes 
through fine, but in this particular instance the response to the server 
needs to include an additional parameter. The RFC document for RADIUS 
specifies that the packet must have attached:

State =     {Magic Cookie from Access-Challenge packet, unchanged}

The packet sent back must be an Access-Request format, as the original, 
but with a different identifier and the same data.

This is as an additional attribute. This is the code I have to construct 
packets sent to the RADIUS server.

   $data=pack("CCCCa*CCCCCCCCa*CCa*CCCCCCCCCCCC",
   1,$thisidentifier,$length/256,$length%256,        // header
   $RA,                        // authcode
   6,6,0,0,0,1,                    // service type
   1,2+strlen($username),$username,            // username
   2,2+strlen($encryptedpassword),$encryptedpassword,    // userpassword
   4,6,$nasIP[0],$nasIP[1],$nasIP[2],$nasIP[3],    // nasIP
   5,3,0,0,0,0                        // nasPort
   );

I would assume that to add an additional attribute, I would just need to 
add an additional line (and alter the data format parameter in the 
pack() call):

   24, <packet data here>               // State (24)

However, I am a little lost on how to get that data in there, and where 
the "Magic Cookie" comes from (as described in the State detail above). 
How do I get this state data? Is it in the response from the RADIUS server?

I understand what the RADIUS server needs, but I'm not 100% sure exactly 
how to:

a) get the state data to send back
b) append this data to the packet

Is anyone able to shed any light on this for me? Has anyone had 
experience with doing an implementation for software that deals with 
Access-Challenge packets? Are there any links anyone can offer that 
might explain how I can accomplish this? I have tried using the RADIUS 
module for PHP but it appears to be a little obscure and different to 
how we have already implemented the current version.

ANY help would be appreciated.

Thanks and regards,
Dwight.