Has anyone attempted to use FDF?

I am trying to read information from an Acrobat form, has anyone written anything that works using FDF? I am trying to read $HTTP_FDF_DATA but it keeps coming up blank. (From example on php.net, under quick ref-> fdf_open) Is there something I have to set on the NT server or in php.ini to read in this special variable, or am I missing a module for FDF? Thanks, Todd