Passing data to a post_action script

[Just Marc]

Hello Ngixers,

Happy sys admin day! (tomorrow)

I am trying to pass data from our PHP authorizer script all the way down 
to post_action, like so:

1. fastcgi - php authorizer script - passing X-Accel-Redirect
2. the file gets sent to the user
3. Now using post_action to run another php script when the request ends

I need a way to transfer information (a key) from the 1st php script 
down to the 2nd php script.  Passing it in a header would be good, any 
other way should also work.

Some stuff I tried that didn't work::

fastcgi_param  PARAM_EXTRA_3    $upstream_http_x_session_id;
or
post_action /done/$upsteram_http_x_session_id

Any suggestions would be greatly appreciated.

Thanks!
Marc