Should I always write implements?

Question

PHP

When you need to inherit a class that implements some kind of interface, I don’t know if implements should be added to the heir.
Example:

interface ObserverInterface
{
    public function execute();
}

abstract class ParentClass implements ObserverInterface
{
}

class ChildClass extends ParentClass // implements ObserverInterface ?
{
    public function execute()
    {
    }
}

I know that this is not necessary for work, but how is it according to the standard?

Answer 1

It just doesn't make sense, because the child class already has all the protected and public methods.