index.php is the main web entry point for the MediaWiki engine. The source code is at svn:trunk/phase3/index.php/[1]. The documentation is at [2]. A brief description is at mw:Manual:Index.php.
variables
edit(documentation: mw:Manual:Global variables,mw:category:MediaWiki configuration settings,mw:category:variables, [3])
In rough order of appeareance:
$mediaWiki $maxLag $action $wgRequest $title $wgTitle $wgOut $wgContLang $wgUseAjax - mw:Manual:$wgUseAjax $IP - mw:manual:$IP ("include path") $dispatcher $wgLoadBalancer $wgServer $wgDisableInternalSearch $wgSquidMaxage $wgEnableDublinCoreRdf $wgEnableCreativeCommonsRdf $wgCommandLineMode $wgUseExternalEditor $wgDisabledActions $wgArticle $wgUser $wgRequest $wgPostCommitUpdateList $wgLoadBalancer