您尚未添加fallback_cb 参数数组的键。您还需要添加theme_location 密钥：

php start $menu = wp_nav_menu(
   array(
      \'menu_class\' => \'reset sf-menu\',
      \'container_id\'=>\'nav\',
      \'echo\'=>0,
      \'fallback_cb\' => \'mytheme_fallback_function\' // ADD ME!!!
      \'theme_location\' => `location_name` // ADD ME, AS DEFINED IN register_nav_menu()
   )
);

$menu = str_replace("\\n", "", $menu);
$menu = str_replace("\\r", "", $menu);
$menu = str_replace("\\t", "", $menu);
echo $menu;
php end

if ( has_nav_menu( \'location_name\' ) ) {
    echo $menu;
} else {
    // Do something else, like wp_list_pages() or wp_page_menu()
}