recurly/recurly-client-php — pull request #107 by dmatora (2014-09-25 06:25:40+00:00)
Title: Provided access to invoice id on manual subscription

Here is how i'm using it
$subscription = new Recurly_Subscription();
...
$subscription_response = $subscription->create();
$href = explode('/', $subscription_response->__get('invoice')->getHref());
$invoice_number= array_pop( $href );