Can't get PHP Session to work with Flash

/**
 *
 * AS snippet
 *
 */
// Create a file reference
var file:FileReference = new FileReference();
// Get the session ID from the cookie
var res:String
= ExternalInterface.call('function(){return document.cookie;}');
var sidStr:String = res.substr(10,res.length);	
// Create a new URL Request
var req:URLRequest = new URLRequest();
req.url = "http://mysite.com/php/session.php";
req.method = URLRequestMethod.POST;
var reqvars:URLVariables = new URLVariables();
reqvars.sid = sidStr;
// Upload File (myFileFieldName is the name of the file in $_FILES var)
file.upload( req, myFileFieldName ); 
 
/**
 *
 * PHP snippet
 *
 */
// Get the session ID and start the session
session_id( $_POST['sid'] );
session_start();
// Return the info to the Flash App as XML for parsing
header("Content-type: text/xml");
$xml = new DOMDocument('1.0', 'iso-8859-1');
$child = $xml->createElement("upload");
$data = $xml->createElement("data");
// Returned XML contains the correct session id
$ssid = $xml->createElement("ssid",session_id());
// But not the user information, and session is unset or
// otherwise destroyed/lost/who knows
$info = $xml->createElement("userInfo",$_SESSION['userInfo']);
$data->appendChild( $info );
$data->appendChild( $ssid );
$child->appendChild($data);
$xml->appendChild( $child ); 
echo $xml->saveXML();
exit();