Warning ao gravar no MySQL

Quando tento gravar os dados de um formulário no BD, retorna esse Warning:

Warning: Unknown: Your script possibly relies on a session side-effect which existed until PHP 4.2.3. Please be advised that the session extension does not consider global variables as a source of data, unless register_globals is enabled. You can disable this functionality and this warning by setting session.bug_compat_42 or session.bug_compat_warn to off, respectively in Unknown on line 0

Essa é a página q grava:

<?php
session_start();
include"config.php";

$qData = "select * from `users` where (`Uid` = '".$_SESSION["Uid"]."')";
$rData = mysql_query($qData) or die(mysql_error());
$aData = mysql_fetch_array($rData);

$Username = $_POST[username];
$Password = $_POST[Password];
$First_Name = $_POST[First_Name];
$Last_Name = $_POST[Last_Name];
$Email_Address = $_POST[Email_Address];
$Country = $_POST[Country];
$Titulo_portf = $_POST[Titulo_portf];
$Cor = $_POST[Cor];
$Avatar = $_FILES["avatar"]["name"];
$tmp_name = $_FILES["avatar"]["tmp_name"];
$Status = $_POST[status];
$t = time();

$pasta = 'avatar/';
$extensao = strtolower(end(explode('.', $Avatar)));
$nome = md5(uniqid()) . '.' . $extensao;

if(move_uploaded_file($tmp_name, $pasta . $nome)){

$q1 = "UPDATE users SET First_Name = '".$First_Name."', Last_Name = '".$Last_Name."',
Email_Address = '".$Email_Address."', Titulo_portf = '".$Titulo_portf."', Cor = '".$Cor."', Avatar = '".$nome."', Country = '".$Country."'

WHERE `Uid` = '".$_SESSION["Uid"]."'";
mysql_query($q1);

header("location: myimages.php");

}
?>

O que está acontecendo?