About Qdig Paths

Overview

Both the Qdig script and the admin.php script need to know a self-referring URL, meaning a URL that points the browser back to the script itself.
Both the Qdig script and the admin.php script need to know how to find the root directory of the gallery tree (1) on the filesystem and (2) as a URL.
The Qdig script needs to know the location of the admin.php script.
The admin.php needs to know the location of the Qdig script.

The self-referring URL is nearly always auto-detected accurately. In the rare case you need to set it, the setting is $qdig_url.
The root directory of the gallery tree as a URL (a.k.a. the base URL path to the images) is set using $url_base_path.
The root directory of the gallery tree as a filesystem path is set using $fs_base_path.
The location of the admin.php script is set using $admin['script_file'], or $admin['full_url'] if that's not sufficient.

(The admin.php script is not essential for running Qdig galleries.)

The self-referring URL is auto-detected.
The root directory of the gallery tree as a URL (a.k.a. the base URL path to the images) is set using $this->web_dir.
The root directory of the gallery tree as a filesystem path is set using $this->base_dir.
The location of the Qdig script is set using $this->qdig_file.