How do I include an external PHP file in a subdir WordPress install?

Question

In you case I’d edited my wp-config.php, and just after

if ( !defined('ABSPATH') )
        define('ABSPATH', dirname(__FILE__) . "https://wordpress.stackexchange.com/");

I’d added

$path = explode( basename( dirname(__FILE__) ), ABSPATH ); 
define( 'MYSITEROOTPATH', $path[0] );

after that, everywhere I need, I’d used

// file.php is name of a file in root directory
include( MYSITEROOTPATH . 'file.php');