Call to undefined function in wp-includes/var.php

Hi,

I have de-activated all plugins and moved back to twenty twelve theme and reset the browser and saved a fresh copy of query.php in wp-includes folder and vars.php but I am still getting this error:

Fatal error: Call to undefined function is_admin() on line 23

on this page:

https://aussiebuilder.com/wp-includes/vars.php

Any ideas?

Whereas other sites I have just give a 403 not allowed message when I try and access the same type of page.