<?      class mysql{         privete static $instance ;//保存实例          //构造函数声明为private， 防止直接创建对象         privete function __construct(){              // 实例化          }          //单例方法， 判断是否已经实例化，只实例化一次          public static function getInstance (){              if(!isset( self::$instance )){                 self ::$instance = new self();              }              return self:: $instance;          }          //防止克隆对象          private function __clone (){              trigger_error ("not allow to clone.");          }          function test(){              echo "test" ;          }      }      $conn = mysql::getInstance ();      $conn->test (); ?>