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();