One ($instance =null;) has been defined above
Judgment (self::$instance instanceof self) is definitely (false)
Why do we still need to judge?
Isn’t it enough to directly assign the current object instance to (self::$instance) and then return the value?
self::$instance =new self($sitename);
return self::$instance;
That’s why we need the singleton mode. If we assign values repeatedly, it will become multiple instances