protected function clean_url($value) {
        global $CFG;

        if (empty($value)) {
            // Nothing.

        } else if (strpos($value, $CFG->wwwroot . '/') === 0) {
            $value = str_replace($CFG->wwwroot, '', $value);

        } else if (strpos($value, '/') !== 0) {
            debugging("Invalid url_select urls parameter: url '$value' is not local relative url!", DEBUG_DEVELOPER);
        }

        return $value;
    }