session_start();

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

 //initiate cart as empty array
    $_SESSION['cart'] = [];
 }

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

 //add product id to session cart
 $_SESSION['cart'][$_POST['add']] =1;
 }

 else if (isset($_POST['remove'])) {

    //remove product id from session cart
    unset($_SESSION['cart'][$_POST['remove']]);
}