Manual talk:Hooks/SkinTemplateOutputPageBeforeExec

When I try to use this hook in an extension, the MediaWiki page shows lots of warnings like this:


 * Warning: Cannot modify header information - headers already sent by (output started at (myexteion).php:14) in includes/WebResponse.php on line 1

Is this hook not suited for use in an extension? How can these warnings be suprssed?