for variables that are sent with GET (<form action="..." method="get")
use
$_GET['variable']
//instead of
$variable
for those with POST
$_POST['variable']
//instead of
$variable
for cookies
$_COOKIE['variable']
//instead of
$variable
or just tu be sure use
$_REQUEST['variable']
//instead of
$variable
for any variable comming from outside your script(form, cookies)