Per Facebook credits documentation.
Get Order
Gets order details for the specified order_id
API
GET https://graph.facebook.com/[order id]?access_token=ACCESS_TOKEN
Note: You must use actual orders when calling the Graph API. Orders from test users will not be processed.
How do we test this call on dev apps? Is the only way to test this by making real Facebook credits purchases on the app? I would rather user $facebook->api_client->getOrderDetails if that is the case.