PHP - Debugging Curl

I'd like to see what the post fields in the request are before I send it. (For debugging purposes). The PHP library (class) I am using is already made (not by me), so I am trying to understand it. As far as I can tell, it uses `curl_setopt()` to set different options like headers and such and then it uses `curl_exec()` to send the request. Ideas on how to see what post fields are being sent?