protected function callbacks(): array
    {
        if (!is_array($this->callbackList)) {
            if (!file_exists($this->configPath . '/xsl-callbacks.ini')) {
                $this->callbackList = [];
            } else {
                $this->callbackList = @parse_ini_file($this->configPath . '/xsl-callbacks.ini');
                if (false === $this->callbackList) {
                    throw new XslCallbackException(
                            'XSL callback in ' . $this->configPath
                            . '/xsl-callbacks.ini contains errors and can not be parsed.'
                    );
                }
            }
        }

        return $this->callbackList;
    }