Regarding PHP Admin control panel design, is iframe a good choice?

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Well lately I've decided to split the big script in admin.php(admin control panel file) into several smaller script files such as admin_index.php, admin_settings.php and admin_createuser.php. I was thinking about this idea of locating all menus/submenus to the left hand side of the page and use an iframe to load info from any admin script files. This way the admin can access everything without having to be directed to a different url. I believe this is what Vbulletin has been doing for its ACP(they may have used javascript though), and I have a few questions to ask:

 

1. Is this a feasible idea, especially if I choose not to use javascript?

2. Is it safe to design admin control panel this way for a PHP script? If not, please lemme know possible security issues?

3. Is it possible to restrict direct access to other admin type script files, but allow access if they are loaded in iframe?

 

Please do lemme know, thanks.