Our server uses the PHP SDK to validate subscription purchases. https://developers.google.com/android-publisher/api-ref/purchases/subscriptions/get
Is it possible to view purchase history on the server side via the SDK, or some endpoint? Basically I'm looking for a way to get the subscription order number.
There is currently no API or endpoint that you can call/use to retrieve subscriptions/purchases history for Google Play.
AFAIK, the expected procedure for this is for developers to keep track of the history on their end by storing and keeping track of the orderIds and purchaseTokens accordingly on their side.