Secure CURL to & handle response from Payment Gateway
Well, security from what type of threat? You need to figure out what you want to protect against before you can protect against it! Since I'm unsure of what you're protecting against, I'll point out relevant setopt flags.


CURLOPT_VERBOSE - This is fine for a developer, but make sure the client doesn't see this returned information. It could lead to a possible exploit. More on the output later on.
CURLOPT_POST - You're using this incorrectly. The value should be true or an alternate HTTP POST.
CURLOPT_RETURNTRANSFER - Using 1 does result in TRUE, but it's easier to read if the explicit boolean is used.


Here are some flags you may way to consider in order to help secure this connection even more. Each can be found in the docs.


bool CURLOPT_FAILONERROR - Just in case the page you fetch is "out of order".
bool CURLOPT_FORBID_REUSE - Force the connection to close on finish, and prevent the reuse of a connection. Also see bool CURLOPT_FRESH_CONNECT.
Make sure you keep bool CURLOPT_SSL_VERIFYPEER set to TRUE.
int CURLOPT_TIMEOUT - Timeout if the connection takes too long.