From PhpCOIN Documentation
An auxpage is a page that has nothing to do with phpCOIN but you want it to appear within phpCOIN's display and have access to all the phpCOIN info. An example would be a client's Domain Control Panel.
Auxpages can contain anything, from html to php code, and are "wrapped" within phpCOIN. It also means that your external pages can access phpCOIN data and variables without having to setup any database connections or do any fancy interfacing.
Auxpages differ from SiteInfo pages in that siteinfo is for pages that display text. SiteInfo pages are basically for pages that form part of phpCOIN, such as the index page. Plain html is the order of the day here.
Create your desired auxpage, save it in /coin_auxpages directory, then call it with auxpage.php?page=pagename_without_DOTphp_extension
The sample /coin_auxpages/auxpage_example.php gives you an idea how to create an auxpage, and even shows how to do it if it should only be accessed by logged in users/admins.