Im confused

Hi Konrad, 'isset' is used to determine that a value is not null.

// we need to get $_GET["id"],
// but only if its not null
if (isset($_GET["id"])) {
    // store it
    $product_id = $_GET["id"];
    // check if $products[$product_id] is not null
    // before using it
    if (isset($products[$product_id])) {
        $product = $products[$product_id];
    }
}
// redirect if $product wasn't successfully created
if (!isset($product)) {
    header("Location: shirts.php");
    exit();
}

Does this help?

It is checking to see if that form parameter has been set. So it is saying if the form field id is set to a value assign that value to a variable product id. Then it essentially does the same for the next part it just uses the product id variable to look up which product it is then set the product variable to whichever product it is in the products array.

So basicaly isset is checking if the product_id od Id is there and if it is, it makes the product as products id and displays the right id from the shirt in the page right?