gistfile1.php

<?php

class Shipping
{
    public function isAvailable()
    {
        // HOOK: allow to add custom logic
        if (isset($GLOBALS['ISO_HOOKS']['shippingIsAvailable']) && is_array($GLOBALS['ISO_HOOKS']['shippingIsAvailable']))
        {
            foreach ($GLOBALS['ISO_HOOKS']['shippingIsAvailable'] as $callback)
            {
                $objCallback = \System::importStatic($callback[0]);
                $available = $objCallback->$callback[1]($this);

                if (true === $available || false === $available) {
                    return $available;
                }
            }
        }

        ...
    }
}