PHP: self:: vs parent:: with extends

I'm wondering what is the difference between using self:: and parent:: when a static child class is extending static parent class e.g.

class Parent {

    public static function foo() {
       echo 'foo';
    }
}

class Child extends Parent {

    public static function func() {
       self::foo();
    }

    public static function func2() {
       parent::foo();
    }
}

Is there any difference between func() and func2() and if so then what is it ?

Thank you

Regards