Manual:Setup.php

This file is included from WebStart.php, provided the  constant is not defined. It must be included after LocalSettings.php.

Description
It does the following:
 * 1) First it changes some configuration settings such as paths variables that aren't customized in LocalSettings.php so that they don't need to be modified if $wgScriptPath is changed or it updates new configuration settings from the backward compatible ones.
 * 2) Next it includes some commonly used files.
 * 3) It starts the PHP session if the session cookie or the user token cookie are present.
 * 4) It creates stub objects for global object variables, those globals will be unstubbed at the first call of one of their method.
 * 5) It runs all functions in $wgExtensionFunctions.
 * 6) And calls some deprecated hooks for backward compatibility: LogPageValidTypes, LogPageLogName, LogPageLogHeader and LogPageActionText.