Woocommerce get order key - php

Question

I have orders in the format [domain]/checkout/order-received/[order_number]/key=[wc-order-key] - how do I get [wc-order-key]?
So far I've done:
add_action('woocommerce_payment_complete', 'custom_process_order', 10, 1);
function custom_process_order($order_id)
{
$order = new WC_Order( $order_id );
$myuser_id = (int)$order->user_id;
$user_info = get_userdata($myuser_id);
$items = $order->get_items();
foreach ($items as $item)
{
$product_name = $item['name'];
$product_id = $item['product_id'];
$product_variation_id = $item['variation_id'];
$product_description = get_post_meta($item['product_id'])->post_content
}
return $order_id;
}

Answer 1

If i understand correctly, you need get order_key by order_id, is it correct?
If so, you can just use WC_Order property:
$test_order = new WC_Order($order_id);
$test_order_key = $test_order->order_key;
Edited
As mentioned indextwo, since Woo 3.0 there new syntax:
$test_order = wc_get_product($order_id);
$test_order_key = $test_order->get_order_key();

Answer 2

2018 updated answer
As WooCommerce 3 changed how property calls were made, the appropriate way to get the same information is:
$order = wc_get_order($order_id);
// Added a check to make sure it's a real order
if ($order && !is_wp_error($order)) {
$order_key = $order->get_order_key();
}
Note that you can easily do the same in reverse: get an order ID from an order key:
$order_id = wc_get_order_id_by_order_key($order_key);