The simplest thing to do is get the FDF data from $HTTP_RAW_POST_DATA. 
(unless you have the server library installed none of the fdf data gets
parsed!) This is typical of  what you get:
%FDF-1.2
1 0 obj
<< 
/FDF << /Fields [ << /V (0)/T (amount0)>> << /V
(0)/T (amount1)>> << /V (0)/T (amount2)>> 
<< /V (0)/T (amount3)>> << /V (0)/T (amount4)>>
<< /V (0)/T (amount5)>> 
<< /V (0)/T (amount6)>> << /V (0)/T (amount7)>>
<< /V (0)/T (amount8)>> 
<< /V (0)/T (amount9)>> << /V /0102 /T
(chase_bk)>> << /V (0)/T (count)>> 
<< /V (0)/T (invtotal)>> << /V (12/21/2000)/T
(sent_ap)>> << /V /Off /T (spec_hand)>> 
<< /V (041232)/T (transit_no)>> << /V (THIS FORM IS NOT
COMPLETE!!!)/T (X)>> 
] 
/F ()>>

>> 
endobj
trailer
<<
/Root 1 0 R 

>>
%%EOF
kill everything before the [ and then parse it down into key value pairs. 

I wrote this to create an FDF, make sure you do a 
header("Content-type: application/vnd.fdf");
before you echo the returned value to the user.
function FDFput(,$FDFpage)
{
 $A = "%FDF-1.2\n1 0 obj\n<< \n/FDF << /Fields [
\n";
 $C = " ] \n"	;
	 if ($FDFpage>"" ) {$C .=" /F
($FDFpage)>>\n";}
	$C .= ">>\n>> \nendobj\ntrailer\n\n<</Root 1 0
R>>\n%%EOF\n";
	$B = "";
 reset($FDFData);
 while (list($key, $val) = each($FDFData))
  {
  if (strlen(trim($val)) > 0 && is_string($key))
   {
	$B .= "<</T ($key) /V (". $val .
")>>\n";
//echo 	"<</T ($key) /V (". $val .
")>>\n";
				}
  }
	return $A.$B.$C;
}
 
It ain't perfect - but it works. (I use HTML for posting to the server,
FDF to the browser)
joe

Here is an easy script to output fdf data to the browser without using the
fdf toolkit or creating an actual fdf file on the server.
By the way acrobat is very picky about line breaks so you must leave the
"\n" in the script. The script reads the variables posted to it
from a form use POST and creates a fdf file from them. The field names
posted to this script must match the field names in the pdf. Acrobat will
ignore any that don't match.

<?php

//path to pdf file
$url="

$values=$HTTP_POST_VARS;

$fdfdata = "%FDF-1.2\n%����\n";
$fdfdata .= "1 0 obj \n<< /FDF ";
$fdfdata .= "<< /Fields [\n"; 

//loop that adds the field names and values
foreach($values as $key=>$val)
	{
	$fdfdata.="<< /V ($val)/T ($key) >> ";
	}

	
$fdfdata .= "]\n";
$fdfdata .= "/F ($url) >>";
$fdfdata .= ">>\nendobj\ntrailer\n<<\n/Root 1 0
R\n>>\n";
$fdfdata .= "%%EOF";

/*** Now we display the FDF data which causes Acrobat to start  ***/

header ("Content-Type: application/vnd.fdf");
print $fdfdata;

?>

You can use javascript in the pdf to read the values from a GET method
posted directly to the pdf. you can see both methods here: 

function parse($file) {
                if
(!preg_match_all("/<<\s*\/V([^>]*)>>/x",
$file,$out,PREG_SET_ORDER))
                        return;
                for ($i=0;$i<count($out);$i++) {
                        $pattern =
"<<.*/V\s*(.*)\s*/T\s*(.*)\s*>>";
                        $thing = $out[$i][1];
                        if (eregi($pattern,$out[$i][0],$regs)) {
                                $key = $regs[2];
                                $val = $regs[1];
                                $key =
preg_replace("/^\s*\(/","",$key);
                                $key =
preg_replace("/\)$/","",$key);
                                $key =
preg_replace("/\\\/","",$key);
                                $val =
preg_replace("/^\s*\(/","",$val);
                                $val =
preg_replace("/\)$/","",$val);
                                $matches[$key] = $val;
                        }
                }
                return $matches;
        }