This is necessary because PHP in Windows does not have any certificates and some *nix systems have outdated or missing ca bundles too.

The order is:
1/ dataroot/moodleorgca.crt always wins - needs to be added manually by admin
2/ php.ini setting "curl.cainfo" is next
3/ on Windows libdir/cacert.pem is used because it does not have any default cert bundles
4/ system default is the last - the previous value, ok for properly configured *nix systems