PHP Session多维数组未被赋值任何值

Question

After a button is being pressed, 3 values are put in into a multidimensional array in PHP. I've checked the values before assigning in the same brackets and it seems to have the correct value. However, when I add the values like this:

if (isset($_POST['add_to_cart'])) {
    $count = count($_SESSION['shopping_cart']);
    echo "Count: $count<br />";

    $_SESSION['shopping_cart'][$count]['product_id'] = $_POST['product_id'];
    $_SESSION['shopping_cart'][$count]['tier'] = $_POST['tier'];
    $_SESSION['shopping_cart'][$count]['division'] = $_POST['division'];
}

The output shows that the array $_SESSION['shopping_cart'] is empty and has no values.

if (empty($_SESSION['shopping_cart'])) {
    echo "Your cart is empty.<br />";
}
else {
    //Display products in cart
    foreach($_SESSION['shopping_cart'] as $id => $product) {
        echo $product['tier'] . $product['division'] . "<br />";
    }
}

I've came to the conclusion that I assign the values in a wrong way. What am I doing wrong? Thanks for the help!

EDIT: forgot to add that the array is already initialized at the beginning!

if(!isset($_SESSION['shopping_cart'])) {
    $_SESSION['shopping_cart'] = array();
}

Answer 1

I forgot to add the session_start(); to the code. That's why the Session array didn't work! Thanks for the help @Will