fdf and pfd functions in PHP

Okay, I'm trying to teach myself how to utilize fdf and insert into pdf and I have a question for y'all. When using pfd forms to submit to an fdf file, the php script uses the variable $HTTP_RAW_POST_DATA as the source to write to the fdf file. If I use an HTML form to do the same thing, what variable would I use? I tried $HTTP_POST_DATA and it returned an empty variable set resulting in an empty fdf file. The code I'm using is as follows: <?php $fdffp = fopen("test.fdf","w"); fwrite($fdffp, $HTTP_POST_DATA, strlen($HTTP_POST_DATA)); fclose($fdffp); ?> Sorry for the basic question but I guess we all have to start somewhere! Cheers, Dave