php - Quickpay API callback: Undefined index: HTTP_QUICKPAY_CHECKSUM_SHA256

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Solution:

SOLVED! It turns out that a security feature in my script regarding the handling of $_POST was blocking the callback from Quickpay - I turned it off, and now it works just fine.

So... as it turns out: $_SERVER["HTTP_QUICKPAY_CHECKSUM_SHA256"] is populated when you get the callback from Quickpay.