Directly from source for
* These functions can be replaced via plugins. If plugins do not redefine these
* functions, then these will be used instead.
* @package WordPress
So, there's your answer, the functions in
pluggable.php are intended to be overridden by Plugins.
Re: load order:
See this post by Konstantin Kovshenin. The relevant points (10-15):
wp_get_active_and_valid_plugins() retrieves the list of all active plugin files for loading and includes them. This is the point where your plugin code gets executed, functions, classes defined, etc.
wp-includes/wp-pluggable-deprecated.php which include functions (and deprecated functions) that can be redefined by plugins. Like
wp_mail() for more advanced mailing,
wp_authenticate() for alternative authentication methods, etc.
wp_set_internal_encoding() is called to set the internal encoding according to the blog_charset option.
wp_cache_postload() is called if object caching is enabled.
- At this point a
plugins_loaded action is fired. This is the very first action (after muplugins_loaded fired before loading the non-multi-site WordPress plugins) that you can hook into, it comes before the init because WordPress has not been initialized yet, at least not fully.
- Plugins are loaded
pluggable.php is loaded
plugins_loaded action is fired
Which is the expected order.
(Note: this all takes place in