Rewritten code based on order class instead of strange arrays.

<?php
// we assume the order class and all the other classes are stored
// in a file that we can then include where we need it
require_once "./orderclass.php";

// tax you pay or not, should be per order not per item
$tax = $_POST['with_gst'] === "1" ? new tax("GST", 5) : new tax();
$order = new order($tax);

if (isset($_POST["submit"])){
    $order->store_id = "store3";
    $order->api_token = "yesguy";
    $order->checkout_id = "chktGLFZQtore3";
    $order->environment = "qa";
    $order->action = "preload";

    foreach($_POST['quantity'] as $key => $quantity) {
        if($quantity === 0) {
            continue; // we will get the next product
        }

        // All products cost 60
        $item = new item($_POST['id'][$key], $_POST['description'][$key], 60, $_POST['quantity'][$key]);
        $order-AddItem($item);
    }

    // Add contact details
    $order->contact_details->first_name = $_POST['bill_first_name'];
    $order->contact_details->last_name = $_POST['bill_last_name'];
    $order->contact_details->email = $_POST['email'];
    $order->contact_details->phone = $_POST['bill_phone'];

    // Add billing details
    $address = new address();
    $address->address_1 = $_POST['bill_address_one'];
    $address->address_2 = $_POST['bill_address_two'];
    $address->city = $_POST['bill_city'];
    $address->province = $_POST['bill_state_or_province'];
    $address->country = "CA"; // only ship to Canada
    $address->postal_code = $_POST['bill_postal_code'];

    $order->billing_details = $address;

    if($_POST['ship_to_billing'] === "1")
    {
        $order->shipping_details = $address;
    } else {
        $order->shipping_details->address_1 = $_POST['ship_address_one'];
        $order->shipping_details->address_2 = $_POST['ship_address_two'];
        $order->shipping_details->city = $_POST['ship_city'];
        $order->shipping_details->province = $_POST['ship_state_or_province'];
        $order->shipping_details->country = "CA";
        $order->shipping_details->postal_code = $_POST['ship_postal_code'];
    }
}

$myObj = $arr;
$myJSON =  json_encode($order);

echo $myJSON;

// Prepare new cURL resource
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, "https://gatewayt.moneris.com/chkt/request/request.php");
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLINFO_HEADER_OUT, true);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_POSTFIELDS, $myJSON);

$array_length = count($arr, COUNT_RECURSIVE);

// Set HTTP Header for POST request
curl_setopt($ch, CURLOPT_HTTPHEADER, array(
    'Content-Type: application/json' ,'Content-Length: ' . strlen($myJSON)
 )
);

// Submit the POST request
$result = curl_exec($ch);

// Close cURL session handle
curl_close($ch);
print_r($arr);
print_r($result);
$response_data = json_decode($result);
print_r($response_data);


$result_data = json_decode($result, true);
print_r($result_data);

if( isset($result_data['response']['error']) &&  ( ! (bool) $result_data['response']['success'] ) ){
    echo 'error message';
}
$token = $result_data['response']['ticket'];
?>