problem with $_POST vars in templates

Hello, I modified a template and added a form in it.
I have a field
<input type="hidden" name="press_action" value="login">

then in my page;

if ($_GET['press_action']) {
	$press_action = $_GET['press_action'];
} elseif ($_POST['press_action']) {
	$press_action = $_POST['press_action'];
}
echo "test".$press_action;

If I try with ?thecat=3&press_action=getlogin, it gives me testgetlogin, so it works.
but if I try with the form inside the template (and so with a post var); it doesn’t write testlogin.

Very strange, uh ?

Any idea ?

Thanks !