edit LocalSettings.php and add:

 if ((!empty($_SERVER['PHP_AUTH_USER']) &&
	!empty($_SERVER['REMOTE_USER'])) ||
	$_COOKIE['fpwiki_en_UserID']) {

        require_once("$IP/extensions/HttpAuthPlugin.php");
        $wgAuth = new HttpAuthPlugin();
        $wgHooks['UserLoadFromSession'][] =
		array($wgAuth,'autoAuthenticate');
 }