PHP 单态设计模式

class Instance {  //静态变量保存全局实例  private static $_instance = null;  //私有构造函数，防止外界实例化对象  private function __construct() {    echo '看我出现几次，就说明我被创建了几次<hr>';  }  //私有克隆函数，防止外办克隆对象  private function __clone() {}  //静态方法，单例模式统一访问入口  static public function getInstance() {    if (is_null(self::$_instance)) {      self::$_instance = new self();    }    return self::$_instance;  }}Instance::getInstance();Instance::getInstance();Instance::getInstance();