PHP - Looking For An Small Advice On Variable Cross Pages

I been having this small curiosity "how to" for some time hope any one could give me an orientation. I m doing an website (from scratch as a self learning project) to "host" a list of movies and at the moment i m using the $_GET variables as navigation indicators (for example http://server/?go=settings, page will display the user settings) and the $_POST variables to pass all kind of form related data. At the moment i m done with all the authentication part (PHP,MySQL + cookies), but what  m missing is a way to tell the user that "well your username or password is wrong", sounds silly i know ... i looked an fancy way of sending an user the warning and i found this small project of jquery called pnotify, where a small ballon will show on the page an message. My problem is to make the ballon apear upon an error of the authentication (if i just paste the code into the body it works). Besides the $_POST or $_GET is there anyway to pass an error code to an object?

This is the code i have for the menu that decides all page navigation:

page.php
Code: [Select]
<?php
session_start();

$mysql = new mysql($dbhost,$dbuser,$dbpwd,$dbname,$seckey);
$auth = new authentication();

require_once(INCLUDES_PATH.'page.header.php');
require_once(INCLUDES_PATH.'inc.actions.php');
require_once(INCLUDES_PATH.'page.footer.php');


action.php
Code: [Select]
<?php
$do = isset($_GET["do"])?$_GET["do"]:"";
switch ($do) {
case "login":
if( isset( $_POST['usr'] ) && isset( $_POST['pwd'] ) ){
$login = $auth->login( $_POST['usr'], $_POST['pwd'], $mysql);
header( "Location:$_PATHS[home]" );
}
else{
require_once(INCLUDES_PATH.'page.login.php');
}
break;
case "logout":
$auth->logout();
header("Location:$_PATHS[home]");
break;
default:
require_once(INCLUDES_PATH.'page.body.php');
break;
}
?>

Thank you in advance.