Fires after WordPress has finished loading but before any headers are sent.
Most of WP is loaded at this stage, and the user is authenticated. WP continues to load on the ‘init’ hook that follows (e.g. widgets), and many plugins instantiate themselves on it for all sorts of reasons (e.g. they need a user, a taxonomy, etc.).
If you wish to plug an action once WP is loaded, use the ‘wp_loaded’ hook below.