{{$page_output}} is a global variable defined by the Horde_Framework. It is used to format the output of dynamically generated pages.
+++ Page Header
$page_output->header(array(
'title' => _("List")
));
++ Page Footer
$page_output->footer();
+++ Adding Javascript
$page_output->addScriptFile('jquery-ui/jquery-1.7.min.js');