单例模式抽象类

abstract class Dao
{
    /**
     * @var
     */
    protected static $instance;

    /**
     * Instance constructor.
     */
    private function __construct()
    {
    }

    /**
     * @return mixed
     */
    public static function GetInstance()
    {
        if (!static::$instance) {
            static:: $instance = new static();
        }
        return static::$instance;
    }

}