Mailgun\Lists\OptInHandler::generateHash PHP Method

    public function generateHash($mailingList, $secretAppId, $recipientAddress)
    {
        $innerPayload = ['r' => $recipientAddress, 'l' => $mailingList];
        $encodedInnerPayload = base64_encode(json_encode($innerPayload));
        $innerHash = hash_hmac('sha1', $encodedInnerPayload, $secretAppId);
        $outerPayload = ['h' => $innerHash, 'p' => $encodedInnerPayload];
        return urlencode(base64_encode(json_encode($outerPayload)));
    }