How can variables or constants be used to determine if a PHP script is included in another script?

To determine if a PHP script is included in another script, you can use a variable or constant specific to the included script and check for its existence in the parent script. By setting a unique variable or constant in the included script and then checking for its presence in the parent script, you can confirm if the script has been included.

// In the included script
define('INCLUDED_SCRIPT', true);

// In the parent script
if(defined('INCLUDED_SCRIPT')) {
    echo 'This script is included.';
} else {
    echo 'This script is not included.';
}