Admin Login Screen Assistance

If this is a basic questions sorry about it, I havn't been on here too long and it's hard to search everywhere.

How would you make it so any .php file of your choosing (like the ones in the root directory, not the admin one) brings up the admin login screen before you can view them? I want to make a few of my root files member-protected but keep them as they are in the folder heirachy, how would I go about doing this?