Use non-contravertial CS

Way back in the beginning of discussions we talked about not using any CS that was specifically omitted from PSR-2 so we don't endorse rival views.

This means

    if (! isset($array[$key)) {
        // ...
    }

Should be written either as 

    if (isset($array[$key) === false) {
        // ...
    }

Or

    if (false !== isset($array[$key)) {
        // ...
    }

I chose the latter simply because that is what is used a few lines up already.