php - Is it a good idea to use $_SERVER['DOCUMENT_ROOT'] in includes?

Question

Is this, for example, a good idea?

require_once($_SERVER['DOCUMENT_ROOT'].'/include.php');

If you have two virtual hosts on the same server, one for live and one for development, with different Apache DocumentRoots, this would avoid having to include absolute paths when the source of the include is unknown, and may be in any directory.

(Note: file paths in the following section are relative to the web root. They would in fact be like /var/www/app/core/init.php, where /var/www/app is the web root)

For instance: I have an /core/init.php which is called using relative paths from places all over the website (/file.php, /dir/file.php or /dir/dir/file.php).

This init.php then includes several function pages, in the fund directory, a subdir of /core (as in /core/func/userfunctions.php).

So, in init.php, I can use the $_SERVER method, because it breaks if I use a relative path and try to call functions from a page like /dir/file.php.

I can't see any problem with it, but in general what could go wrong?

Answer 1

I've seen cases where $_SERVER['DOCUMENT_ROOT'] is not set or is not what you would expect (i.e. not set in CLI or old IIS, or invalid in certain CGI setups).

For that reason you can use dirname(__FILE__) to obtain the path of the script that line is called in. You can then reference relative paths from there e.g.

include dirname(__FILE__) . '/../../other/file.php';

I go with the above method when the directory structure of the files is known and is not subject to change.

If DOCUMENT_ROOT is not available, the following is a suitable replacement:

substr($_SERVER['SCRIPT_FILENAME'], 0, -strlen($_SERVER['SCRIPT_NAME']));