A user taps.
You get paid.
Here is everything in between.

<iframe
  src="https://newoffer.178.105.95.44.nip.io/offerwall?key=YOUR_SITE_KEY&user=USER_ID"
  width="100%" height="720"
  frameborder="0"></iframe>

# the moment the task is completed
POST https://your-app.com/offerwall/credit

user_id     = USER_ID
payout      = 0.4200
offer_id    = 37346175
transaction = a4f1c2...e21d
country     = US
signature   = md5(user_id + payout + secret)

// credit only if the signature matches
$expected = md5($userId . $payout . $secret);

if (!hash_equals($expected, $signature)) {
    return 'BAD SIGNATURE';
}
if (Credit::where('transaction', $txid)->exists()) {
    return 'OK';   // already credited, never pay twice
}
$user->credit($payout);
return 'OK';

# the same catalogue as JSON, to build your own UI
GET https://newoffer.178.105.95.44.nip.io/api/v1/offers?key=YOUR_SITE_KEY

// ->
[{ "id": 37346175,
   "title": "Credit Karma US",
   "payout": "7.85",
   "countries": ["US"],
   "devices": ["android", "ios"] }]