Ability to verify a purchase on an own server

hey anjab, you should take a look, if that still fits your needs. I extended the BillingCache, that it stores the signature of each purchase as well.
This is needed to verify the gplay purchase as you can see for example in this lib:
https://github.com/mgoldsborough/google-play-in-app-billing-verification/blob/master/library/GooglePlay/InAppBilling/GooglePlayResponseValidator.php#L64

I used some functionalities and statics of your classes to make it fit, I hope that's how you would've done it ;)