MediaWiki:Common.css

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public static function onSkinTemplateNavigation( SkinTemplate &$sktemplate, array &$links ) {
    $links['namespaces'][] = array(
        'class' => 'test', // a css class, if you want
        'text' => 'Testtab', // That's the text visible to the user, you might want to use a message key, instead of a hardcoded text
        'href' => 'test', // That's the link the tab will redirect to, when clicked
    );
}