Magento – how to add credit card details input fields in Checkout Page when creating a new payment method gateway

Im not sure if the payment platform you are talking about is something self build or a payment platform that doesn't have a Magento module.

If we're talking about an existing payment platform that doesn't have a Magento module you're wise to consult their documentation on how to transfer the data to them. They should have that. It depends on their architecture what the best method is.

If you are talkin about a self build platform this might help a little: I think you're best of using cUrl to send the initial data to the platform you're redirecting to. Sending the data while redirecting makes it vulnerable to tampering.

define('SECRET_SALT', 'randomstring');

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, "http://www.example.com/createtoken");
curl_setopt($ch, CURLOPT_HEADER, 0);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, http_build_query(array(
    'amount' => 5.00,
    'orderid' => '1000000007',
    'checksum' => hash('sha512', SECRET_SALT . '5.00' . '1000000007'),
), '', '&'));

$token = curl_exec($ch);
curl_close($ch);

header('location: http://www.example.com/redirectpage.php?token='.$token);

This will first send all the information to the platform that processes the transaction, returns a token that you include when you redirect the user. On this custom build payment platform you can then use the token to retrieve the information you previously posted to the platform.

Let me know if you have any more questions or information. Also, the method discribed above is in no way 100% secure, you might need to look into building extra checks to make sure the right amount is payed and linked to the right order

In this page of the docs you can find how to add a custom form.

That tutorial you linked can you also use to add fields to other parts of the shipping step btw. It doesn't have to be in the shipping address form.