The fdf_open() function opens a file with form data. This file must contain the data as returned from a PDF form or created using fdf_create() and fdf_save().
You can process the results of a PDF form POST request by writing the data received in $HTTP_FDF_DATA to a file and open it using fdf_open(). Starting with PHP 4.3 you can also use fdf_open_string() which handles temporary file creation and cleanup for you.
See also fdf_open_string(), fdf_close(), fdf_create() and fdf_save().