Extension:Header/1.0

This is.

Code
Header.php: '.$wgHeader; #Define $wgheader as html and put it at the top of the page. $wgExtensionCredits['validextensionclass'][] = array(      'path' => __FILE__,       'name' => 'Header',       'author' =>'iggyvolz',        'url' => 'https://www.mediawiki.org/wiki/Extension:Header',        'description' => 'Creates an HTML header on all Mediawiki pages.',       'version'  => 1.0,       );

Readme
Installation instructions are avaliable on the main page.

Changelog
Initial version. Should be compatible with all versions of Mediawiki.