null is a failure value coming from key():  "If the internal pointer points beyond the end of the elements list or the array is empty, key() returns null."

No, I get that :)
Your initial comment lists null twice though, once as success value, and once as failure value.

Well it was meant as


@return {string|float|int|bool|null} {scalar on success, or null on failure}.

I don't think we usually split values out of the initial type definition into the description right?

oh boy
Nevermind me then. It's Friday...
but float and bool are still not valid types, so those should go