授权标头没有传入php curl是使用CURLOPT_HTTPHEADER

I have come around a strange issue that when I am passing authorization header like below,

CURLOPT_HTTPHEADER => array(
                "Authorization: Basic XXXXXXXXXX",
                "Content-Type: application/json",
            ),

If I send like this then Authorization is not showing in request header. But If I have Auth details like below,

CURLOPT_USERPWD=>"xxxxx:xxxxxx",

it will adding required Authorization header as I needed in htpp request.

So what is the issue in this? why it is not simply work as given in first code snippet? Also I am not able to send custom header, custom headers are also not available in request body.