Firstly, the index operator ([]) is not only an array operator,
but also a string operator[1], so it makes sense to treat [] on
strings as string operations.

Whether the empty index operator should be supported for strings
had been discussed on internals[2], and IIRC there had been
consensus to not allow it at all. I'm not sure if using the index
operator on strings with string indixes has explicitly been
discussed, but I would rather not treat this differently, to avoid
subtle differences (e.g. $str['1'] vs $str[1]). Instead the
current behavior looks fine for me, because it uses PHP type
juggling rules to convert a string index to a numeric index (in
this case (int)'a' => 0).

So, in my opinion, this is not a bug.

[1] <http://php.net/manual/en/language.types.string.php#language.types.string.substr>
[2] <http://news.php.net/php.internals/91200>

This behavior is due to three factors:

a) $str['a'] behaves as $str[(int) 'a'], i.e. $str[0], together with an "Illegal string offset" warning.
b) Assigning a string longer than 1 to a string offset, will assign the first character of the string.
c) Assigning an offset past the end of a string will extend the string to that offset with whitespace and then perform the assignment.

As such, the behavior is correct under the current semantics. However, it may be time to reevaluate some of those, e.g. make a) a hard error or make b) throw a warning.