一：单例模式的三大原则（1）构造函数需要标记为非public(防止外部使用new操作符创建对象),单例类不能在其他类中实例化，只能被自身实例化。（2）拥有一个保存类的实例的静态成员变量$_instance (3)拥有一个访问这个实例的公共静态的方法二：实现代码
      '127.0.0.1',        'user' => 'root',        'password' => '',        'database' => 'video',    );    private function __construct() {    }    static public function getInstance() {        #判断这个变量是否存在        if(!(self::$_instance instanceof self)) {            self::$_instance = new self();        }        return self::$_instance;    }    #连接数据库    public function connect() {        if(!self::$_connectSource) {            self::$_connectSource = @mysql_connect($this->_dbConfig['host'], $this->_dbConfig['user'], $this->_dbConfig['password']);                if(!self::$_connectSource) {                throw new Exception('mysql connect error ' . mysql_error());                //die('mysql connect error' . mysql_error());            }                        mysql_select_db($this->_dbConfig['database'], self::$_connectSource);            mysql_query("set names UTF8", self::$_connectSource);        }        return self::$_connectSource;    }}/*$connect = Db::getInstance()->connect();$sql = "select * from video";$result = mysql_query($sql, $connect);echo mysql_num_rows($result);var_dump($result);*/