Changing map file parameters via URL

[John Cartwright]

Hello All,

I'm trying to modify an existing layer on a per request basis using
mapserver CGI.  I have a postgis layer defined as:

   LAYER
      NAME US
      ...
      FILTER "iso_2digit = 'US'"
   END

and I am trying to modify the FILTER element by including:

map_US_FILTER=iso_2digit='CA'

in the request.  Can someone point out what I'm doing wrong?  (This is
version 5.0 on MacOSX 10.4)

Thanks!

--john