public static function get_undeletable_block_types() {
        global $CFG;
        $undeletableblocks = false;
        if (isset($CFG->undeletableblocktypes)) {
            $undeletableblocks = $CFG->undeletableblocktypes;
        }

        if (empty($undeletableblocks)) {
            return array();
        } else if (is_string($undeletableblocks)) {
            return explode(',', $undeletableblocks);
        } else {
            return $undeletableblocks;
        }
    }