Hi!
The only concern I have is that support of negative indexing will break
symmetry with (proper) arrays, where we cannot support negative indexing.
I think that was the main source of objections to this proposal in the
past. However, as one might say, string offsets are already not
symmetrical to array offsets - e.g. you can do $array["foo"] but not
$string["foo"].
However this is unlikely to be a significant issue. (I do however play with
the thought of reutilizing the obsolete {} indexing syntax to act as offset
indexing instead.)
I think it may be a good idea to start recommending using {} for string
offsets and [] for arrays. While both syntaxes may work for strings, the
intent (and expected handling of negatives, in this case) is much
clearer when different syntax is used.