Returns the base path (i.e., directory) of the Backdrop installation.
base_path() adds a "/" to the beginning and end of the returned path if the path is not empty. At the very least, this will return "/".
- http://example.com returns "/" because the path is empty.
- http://example.com/backdrop/folder returns "/backdrop/folder/".
includes/ common.inc, line 3318
- Common functions that many Backdrop modules will need to reference.