It doesn't do any control of what one is putting into this array. One can write some incompatible structure there such as `['a' => ['b' => 'c']]` so this interface is misleading since it gives you false safety.

I love concatenation with comma part and the way how `__toString()` works in this case. I understand that without it syntax will be much worse but still it looks like a part of good implementation, not interface.