Could be useful if you create a custom - variable - value script for some local variable settings. In another open-project (Timthumb) where sometimes I partecipate in small development and bug fix all the variable are smart: if there are alredy declared in the custom file the it uses this value, otherwise use the standard ones:
//Load a config file if it exists. Otherwise, use the values below
if( file_exists(dirname(__FILE__) . '/timthumb-config.php')) require_once('timthumb-config.php');
if(! defined('DEBUG_ON') ) define ('DEBUG_ON', false);
e.g. and then write in the config file :
define ('DEBUG_ON', true);
In this way with a little piece of code everybody can personalize the values just editing one single user-created-files and don't edit the new files at each release just to set up a couple of numerical values.
Mine is just a proposal, but it's an interesting way of dealing with custom variable (or also functions...)