I have seen a lot of Drupal modules where requiring php files is done like this
$module = 'my_module';
require_once(DRUPAL_ROOT . '/' . drupal_get_path('module', $module) . '/test.php');
While this is totally right i still encourage you to use special function for this called “module_load_include“.
module_load_include('php', 'my_module', 'test');