It seems that the following script will not work properly with the session_start();

<?php

session_start();

//path to pdf file
$url="http://www.some_url.com/form.pdf";

$values=$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;

?>