add_cart.php

<?php

session_start();
$p_id = $_POST['id'];
$qty = $_POST['qty'];

if (!isset($_SESSION['cart'])) {

    $_SESSION['cart'] = [];
}

if (isset($_POST['add_to_cart'])) {

    if (isset($_SESSION['cart'][$p_id])) {

        $_SESSION['cart'][$p_id] += $qty;
    } else {
        $_SESSION['cart'][$p_id] = $qty;
    }
}
if (isset($_POST['delete'])) {
    $p_id = $_POST['id'];
    if (isset($_SESSION['cart'][$p_id])) {

        unset($_SESSION['cart'][$p_id]);
    }

}

if (isset($_POST['update'])) {
    $p_id = $_POST['id'];

    $qty = $_POST['qty'];

    unset($_SESSION['cart'][$product_id]);

    $_SESSION['cart'][$p_id] = $qty;
}

header('location:cart.php')

    ?>