You can process the results of a PDF form POST request by writing the data recieved in $HTTP_FDF_DATA to a file and open it using fdf_open().
The fdf_open() function opens a file with form data. This file must contain the data as returned from a PDF form. Currently, the file has to be created 'manually' by using fopen() and writing the content of HTTP_FDF_DATA with fwrite() into it. A mechanism like for HTML form data where for each input field a variable is created does not exist.