Extending "SimpleXMLElement" did not work as expected

# Bug report

```SimpleXMLElement->addChild()``` will return "static" and not "SimpleXMLElement"

### Code snippet that reproduces the problem

https://phpstan.org/r/a05cfee2-e145-4812-9850-a4bb2f858376

https://3v4l.org/USJWu

### Expected output

no error