Add support for bitwise string assign ops

[bwoebi]

$str[$off] OP= $x; equivalent to $str[$off] = $str[$off] OP $x;

Fixes fatal error for |=, ^= and &= on string offsets. Other people may feel free to add support for other ops, but I don't see any real sense in it (at least as long as we require the equality above to hold).
Changing that would need a RFC, hence this PR is only strictly doing what it does on e.g. array access too.

[weltling]

@bwoebi please add some tests.

Thanks.

[nikic]

For the record I'm -1 on this because imho we should either support all the operations that are supported in the long form or none of them, not something in between.

[weltling]

@nikic, yeah, you're right ... my thought was either that to support the feature that makes sense by the meaning at least is acceptable, the other cases are still errored anyway.

An RFC were probably something standardizing the behavior, by the research some more useful cases can come out. This is pretty good for the overall specification. So I have to concur with your position. This PR is incomplete and needs an RFC which will produce the basis for the language specification.

Thanks.

[php-pulls]

Comment on behalf of cmb at php.net:

Since this PR appears to be abandoned, and there is no respective RFC, I'm closing this PR for now.