Expanded exmple of the order/cart

<?php

class token {
        function __construct($data_key = null, $issuer_id = null) {
                $this->data_key = $data_key;
                $this->issuer_id = $issuer_id;
        }
        public $data_key;
        public $issuer_id;
}

class recur {
        public $bill_now;
        public $recur_amount;
        public $start_date;
        public $recur_unit;
        public $recur_period;
        public $number_of_recurs;
}

class item {
        function __construct($produc_code = null, $description = null, $unit_cost = null, $quantity = null) {
                $this->product_code = $produc_code;
                $this->description = $description;
                $this->unit_cost = $unit_cost;
                $this->quantity = $quantity;
                // We assume that items have the same page url, just the product code will tell which
                // item will be shown. if you need to change this, you can always do it with
                // $item->url = "https://custom.url.exmple.com";
                $this->url = "https://example.net/item?id=${produc_code}";
        }

        public $url;
        public $description;
        public $product_code;
        public $unit_cost;
        public $quantity;
}

class tax {
        function __construct($description = null, $rate = null, $amount = null) {
                $this->description = $description;
                $this->rate = $rate;
                $this->amount = $amount;
        }

        public $amount;
        public $description;
        public $rate;
}

class cart {
        public $items = array(); // for item objects
        public $subtotal;
        public $tax;
}

class contactdetails {
        public $first_name;
        public $last_name;
        public $email;
        public $phone;
}

class address {
        public $address_1;
        public $address_2;
        public $city;
        public $province;
        public $country;
        public $postal_code;
}

class order {
        function __construct(tax $tax) {
                // just to simply init things for you
                $this->recur = new recur();
                $this->cart = new cart();
                $this->cart->tax = $tax; // assign the tax to cart
                if(is_numeric($tax->rate)) {
                        // update _tax_rate if we have a numeric value
                        $this->_tax_rate = floatval($tax->rate) / 100;
                }
                $this->contact_details = new contactdetails();
                $this->shipping_details = new address();
                $this->billing_details = new address();
        }

        // varaibles used to store internal data not part of json
        private float $_tax_rate = 0.0;
        private float $_subtotal = 0.0;
        private float $_taxtotal = 0.0;

        // public values part of the json
        public $store_id;
        public $api_token;
        public $checkout_id;
        public $txn_total;
        public $environment;
        public $action;
        public $token = array(); // for token objects
        public $ask_cvv;
        public $order_no;
        public $cust_id;
        public $dynamic_descriptor;
        public $language;
        public $recur;
        public $cart;
        public $contact_details;
        public $shipping_details;
        public $billing_details;


        // function to add items to the orders cart and update tax amount, cart subtotal and order txn_total
        // we have a type verifiction that we just have items.
        function AddItem(item $item) {
                $this->cart->items[] = $item;
                $total = intval($item->quantity) * floatval($item->unit_cost);
                $this->_subtotal += $total;
                $this->_taxtotal += $total * $this->_tax_rate;

                // store the values to strings, we don't do any rounding
                $this->cart->subtotal = number_format($this->_subtotal, 2);
                $this->cart->tax->amount = number_format($this->_taxtotal, 2);
                $this->txn_total = number_format(floatval($this->_subtotal) + floatval($this->_taxtotal), 2);
        }

        // function to add tokens to order token, we have type verification and we ensure we don't of
        // mistake replace already added tokens.
        function AddToken(token $token) {
                $this->token[] = $token;
        }

}

// first we need to decide the tax
$tax = new tax("Tax", "13.00");

// we create a new empty order with the tax we had decided for
$order = new order($tax);

// we add two tokens
$order->AddToken(new token("1234", "645sddfvdrt4tefd"));
$order->AddToken(new token("5678", "645sddfvdrt4tefd"));

// we add two items, we do not update other values in this example
$order->AddItem(new item("1", "item 1", "100", "1"));
$order->AddItem(new item("2", "item 2", "200", "1"));

// it's possible to store item as a variable and then add to the
// order:
$item = new item("3", "item 3", "50", "2");
$item->url = "https://custom.url.exmple.net/olditems/33";
$order->AddItem($item);

echo json_encode($order, JSON_PRETTY_PRINT);
echo "\n";

?>