How to include WordPress functions in custom .php file?

Question

How can I include WordPress functions in a custom .php file?

In detail: I have a directory under my theme (Constructor) named reports. These contain

Answer 1

Well if someone has newer PHP versions installed (ver >= 5.5.x) then they can also try the below code in the root script in WordPress website directory itself:

Or

I guess this is a more direct and clean approach and doesn't involve manually adding slashes and changing diretories by ...

Hope this helps someone.