http://php.net/manual/it/language.oop5.late-static-bindings.php
Basically, it boils down to the fact that the self keyword does not follow the rules of inheritance. self always resolves to the class in which it is used. This means that if you make a method in a parent class and call it from a child class, self will not reference the child as you might expect.
Late static binding introduces a new use for the static keyword, which addresses this particular shortcoming. When you use static, it represents the class where you first use it, ie. it ‘binds’ to the runtime class.