$page_output is a global variable defined by the Horde_Framework. It is used to format the output of dynamically generated pages.
$page_output->header(array( 'title' => _("List") ));
$page_output->footer();
// $page_output->addScriptFile($name_of_file_relative_to_base_of_js_directory, $app_name); // Example, for Horde: $page_output->addScriptFile('login.js', 'horde');
// The file MUST be web accessible. $page_output->addScriptFile( new Horde_Script_File_External($relative_URI_to_JS_file) );
$page_output->addInlineScript( $javascript_code, true );
Depending on the presentation type (smartmobile, dynamic, basic), PageOutput automatically includes some appropriate javascript environment.
In Horde 5, dynamic mode is based on PrototypeJs and smartmobile is based on jquery-mobile.
See Ajax Application Docs for more details on available environment.