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

I use this method to load WordPress environment outside WordPress.

  if (file_exists($_SERVER['DOCUMENT_ROOT'] . '/wp-load.php')) {

      /** Loads the WordPress Environment and Template */
      require_once($_SERVER['DOCUMENT_ROOT'] . '/wp-load.php');

  }