Sends a confirmation request email when a change of user email address is attempted.

Sets up global user vars.

Updates all user caches.

Updates user meta field based on user ID.

Updates user option with global blog capability.

Determines whether the given username exists.

Checks whether a username is valid.

Authenticates the user using an application password.

Authenticates the user using the WordPress auth cookie.

Authenticates a user using the email and password.