If "curl.cainfo" is not set (or you are using PHP older than 5.3.7, where the option was introduced), libphutil uses the "default.pem" certificate authority bundle when making HTTPS requests with cURL. This bundle is extracted from Mozilla's certificates by cURL:

http://curl.haxx.se/docs/caextract.html

If you want to use a different CA bundle (for example, because you use self-signed certificates), set "curl.cainfo" if you're using PHP 5.3.7 or newer, or create a file (or symlink) in this directory named "custom.pem".

If "custom.pem" is present, that file will be used instead of "default.pem".