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Tip:

If your code is running on multiple servers with different environments (locations from where your scripts run) the following idea may be useful to you:

a. Do not give absolute path to include files on your server.
b. Dynamically calculate the full path (absolute path)

Hints:
Use a combination of dirname(__FILE__) and subsequent calls to itself until you reach to the home of your ‘/index.php’. Then, attach this variable (that contains the path) to your included files.

One of my typical example is:

< ?php
define('__ROOT__', dirname(dirname(__FILE__)));
require_once(__ROOT__.'/config.php);
? >

instead of:
< ?php require_once('/var/www/public_html/config.php); ? >

After this, if you copy paste your codes to another servers, it will still run, without requiring any further re-configurations.

bimal en PHP: require_once – Manual.