Hi everybody i'm new in php and MySQL and i got the following error when running a page

Notice: Undefined index: item in d:\apache\www\prueba.php on line 4

Notice: Undefined index: cantidad in d:\apache\www\prueba.php on line 5


the initial code of the page prueba.php is:
session_start();
session_register('itemsEnCesta');
$item=$_POST['item'];
$cantidad=$_POST['cantidad'];
$itemsEnCesta=$_SESSION['itemsEnCesta'];
if ($item){
if (!isset($itemsEnCesta)){
$itemsEnCesta[
...
I think it shoulb be a configuration problem, but i don't know where to start.
I run it at my local server apache, MySQL and php, but in a web server it does correct http://www.webestilo.com/php/php12e.phtml that's where i took the example, pls help me :(