I don't understand the meaning of judging whether it is a static instance? ?

Question

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;

Answer 1

That’s why we need the singleton mode. If we assign values ​​repeatedly, it will become multiple instances