PHP configuration file processing class code

/*
* 来源: http://www.xuehuwang.com/
* 作者: 雪狐博客
* 应用: 用于处理配置数据
* 搜集整理：bbs.it-home.org
**/
    final class Config {
    private $data = array();

    public function get($key) {
        return (isset($this->data[$key]) ? $this->data[$key] : NULL);
    }

    public function set($key, $value) {
        $this->data[$key] = $value;
    }

    public function has($key) {
        return isset($this->data[$key]);
    }

    public function load($filename) {
        $file = DIR_CONFIG . $filename . '.php';

        if (file_exists($file)) {
            $cfg = array();

            require($file);

            $this->data = array_merge($this->data, $cfg);
        } else {
            exit('Error: Could not load config ' . $filename . '!');
        }
    }
}