<?php
session_start();

// add cart to session
$product_id = 15;
$qty = 1;
$_SESSION['cart'][$product_id] = $qty;

// update cart

$product_id = 15;
$qty = 2;
$_SESSION['cart'][$product_id] = $qty;

// remove item from cart
$product_id = 15;
unset($_SESSION['cart'][$product_id]);

// check cart

if (!EMPTY($_SESSION['cart'])) {
    $cart = $_SESSION['cart'];
    foreach ($cart as $k => $v) {
        echo 'Product ID: ' .$k;
        echo '<br>';
        echo 'Quantity: ' . $v;
        echo '<br>';
    }
} else {
    echo 'Cart is empty';
}
?>