Manual:User.php

Description
User.php implements the User class, which encapsulates all of the user-specific settings (user_id, name, rights, password, email address, options, last login time). Client classes use the getXXX functions to access these fields. These functions do all the work of determining whether the user is logged in, whether the requested option can be satisfied from cookies or whether a database query is needed. Most of the settings needed for rendering normal pages are set in the cookie to minimize use of the database.

createAndPromote.php uses User::newFromName, User::addToDatabase, and User::saveSettings.

Other methods
There are a whole host of other ones.