function zendesk_update_order($order_id)
{
    $orders = new WC_Order($order_id);
    $items = $orders->get_items();
    $set_plan = false;
    $plan = '';
    if (!empty($items)) {
        foreach ($items as $item) {
            $product_variation_id = $item['variation_id'];
            if ($product_variation_id == 2400) {
                $plan = 'Starter';
                $set_plan = true;
                break;
            } elseif ($product_variation_id == 2401) {
                $plan = 'Standard';
                $set_plan = true;
                break;
            } elseif ($product_variation_id == 2402) {
                $plan = 'Platinum';
                $set_plan = true;
                break;
            } else {
                //THIS ORDER HAS NO ITEMS WITH PACKAGE PLAN
                $plan = '';
            }
        }
        if ($set_plan == true) {
            //STORE PURCHASED PACKAGE PLAN FOR FUTURE REFERENCE
            update_post_meta($order_id, 'Plan', $plan);
            //START PROCESSING ZENDESK REQUESTS
            zendesk_process_user($order_id);
        }
    }
}
<?php

echo zendesk_process_user($_GET['order_id']);