I have a non-SharePoint site, which is written in PHP, and I'd like to display an App Page from a SharePoint site on the non-SP site by loading it inside an iFrame.

The problem is that the entire SP site requires authentication for any sort of access, and I don't want to give out the login credentials for users of the non-SP site. Is there any way I can have the non-SP site securely authenticate the iFrame to view the SP site without the users ever having access to the login credentials?

Thanks for any help or pointers!