Consolidate UNSET_DIM handling for string offset error

The immediate error here is the nested indexing in write context,
the fact that it's ultimately wrapped in an unset() doesn't matte

Consolidate UNSET_DIM handling for string offset error

The immediate error here is the nested indexing in write context,
the fact that it's ultimately wrapped in an unset() doesn't matter.
Same as $str[0][0] += 1 will throw "Cannot use string offset as an
array", so should this case.

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Deprecate autovivification on false

Deprecate automatically converting "false" into an empty array
on write operands. Autovivification continues to be supported
for "null" values, as

Deprecate autovivification on false

Deprecate automatically converting "false" into an empty array
on write operands. Autovivification continues to be supported
for "null" values, as well as undefined/uninitialized values.

RFC: https://wiki.php.net/rfc/autovivification_false

Closes GH-7131.

Co-authored-by: Tyson Andre <tysonandre775@hotmail.com>
Co-authored-by: Nikita Popov <nikita.ppv@gmail.com>

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Fix missing error for single index unset on wrong type

The error was correctly thrown for a nested index, but not for a
single index. Make sure both have the same behavior.