InApp Billing: Fetch Order Details and verify transactions on MY server?

I have made an Android app for selling digital content where items can be purchased using in-app-billing. I need my server to be aware of the purchase. The decision to deliver app-specific data should be made on the server, not in the client app.

E.g.

User buys item X from Android Market.
Transaction data Y is sent automatically to the server. (Which includes Order ID, Customer's First Name and Email)
Server delivers content if Y is valid.
Server returns the file as link to the client.
Client is redirect to page inapp. Says "Thanky for your purchase. Click Here to download the file"
How can this be accomplished? How do I verify that transaction data coming from the Android client (presumably originating from Google servers) is not fake?

Google Server -> Android client -> The server -> Android client

Perhaps this is more of a PHP question than anything else. The PHP function I'm using

// Fetch all the order details from App
$products = array();
$_name = $order->billing_first_name.' '.$order->billing_last_name;
$_email = $order->billing_email;
$lastOrderId = $order->id;