In what situations should constants be used instead of variables for defining file paths in PHP?

Constants should be used instead of variables for defining file paths in PHP when the path is fixed and should not be changed during the execution of the script. Using constants ensures that the path remains consistent and prevents accidental changes. This is particularly useful when dealing with include files, configuration files, or other resources that need to be accessed throughout the application.

define('INCLUDE_PATH', '/path/to/includes/');
include(INCLUDE_PATH . 'header.php');