Be specific when you're asking a question, which part of the API you need help? I can't help, but others who've used the API might able to help if your question is specific enough.
What are you trying to do/achieve using this API? It's possible you fell on XY problem.
Just need help to understand
$callback_url = "
https://site.com/IPN.php?user=2450";
If I don't declare any ?user=xx value will the following data still get sent back to my callback URL?
The following details will be sent to your callback URL upon receiving payments, Payment notification are sent every minute up until the transaction gets 3 confirmations.
$address = $_POST["address"];
$amount = $_POST["amount"];
$confirmations = $_POST["confirmations"];
$hash = $_POST["hash"];
$auth_hmac = $_POST["auth_hmac"];
Thank you