Curl requires a certificate to do full verification of the host and peer through https. Essentially this application was released in this mode which is less secure to work on any system even if that cert is in accessible or not installed on the host.. To specify your certificate and to turn on the validation process you would do something similar to:
curl_setopt($connection, CURLOPT_SSL_VERIFYPEER, 1);
curl_setopt($AuthNetConn, CURLOPT_CAINFO, "/path/to/cert/ca-bundle.crt");
in Windows:
curl_setopt($ch, CURLOPT_CAFILE, 'C:\path\to\cert\ca-bundle.crt');
If you have any problems you can always look in the Curl_Error method which will return any issues with the cert.
The cert is available through the release of Curl in PHP at http://curl.haxx.se You can find the certificate file in the lib directory of the release.
Regards,
Barry R.