How to include core files into plugin

Most of the time, if you are doing things right you don’t have to include Core files. If you are doing things right they will already be loaded for you. In those case where they aren’t loaded, trying to load files with “directory traversal” paths is the wrong way. The Core constants ABSPATH and WPINC should give you the base path the the includes directory.

var_dump(ABSPATH . WPINC);

Just tack on the rest of address.

The Core function includes_url() will help if you need the URL instead of the path.