I want to be able to change one variable and then it will affect something for all users on all pages.
Is there a way to do this without have to manually edit the variable every time I want to change it?
I also don't want to use MySQL because it seems like a waste for just one thing.

It's just an on/off switch that I want to be on the site that the admin should be able to switch.

Save the state or variable value to a flat file.

it can be an included php file, a text file, a XML file etc.

Well chris... this is still a open/edit solution. But to be honest is the only way