PHP - Json is empty after converting object - php

Question

So basically I want to have an Object Class with a bunch of setters that I can manipulate and then convert straight to JSON. In the past I was using an already existing JSON as model(json->object->json). Now I want to just have object->json.
But at the moment I get an empty array when I try to use json_encode.
Also had some trouble with my $index that I'm using because I can have multiple numerical indexes inside "items"
PHP:
<?php
namespace ProjectApi\Models;
use ArrayObject;
use stdClass;
class MagentoInvoice
{
public $capture = true;
public $notify = true;
public function __construct()
{
}
public function setCapture($capture)
{
$this->capture = $capture;
}
public function getCapture()
{
return $this->capture;
}
public function setNotify($notify)
{
$this->notify = $notify;
}
public function getNotify()
{
return $this->notify;
}
public function setItems($itemsCount)
{
$i = 0;
while($i < $itemsCount)
{
$this->items = new stdClass();
$i++;
}
}
public function getItems()
{
return $this->items;
}
public function setProductQty($index, $qty)
{
$this->items->$index->qty = $qty;
}
public function getProductQty($index)
{
return $this->items->$index->qty;
}
public function setProductOrderItemId($index, $id)
{
$this->items->$index->order_item_id = $id;
}
public function getProductOrderItemId($index)
{
return $this->items->$index->order_item_id;
}
}
JSON
{"capture":true,"items":[{"order_item_id":123,"qty":1},{"order_item_id":321,"qty":1}],"notify":true}
$body = new MagentoInvoice();
$body->setCapture(true);
$body->setNotify(true);
if($firstProductId != null && $secondProductId != null)
{
$body->setItems(2);
$body->setProductQty(0, 1);
$body->setProductOrderItemId(0, $firstProductId);
$body->setProductQty(1, 1);
$body->setProductOrderItemId(1, $secondProductId);
}
print_r(json_encode($body));