[wleeaf]

Q	A
Branch?	1.x
Bug fix?	yes
New feature?	no
Deprecations?	no
Issues	-
License	MIT

ext/mbstring encodes and decodes UCS-2 as big-endian. The polyfill passes the name to iconv unchanged. glibc's iconv treats UCS-2 without a BOM as host byte order, so on x86 the polyfill produces and reads little-endian data:

bin2hex(mb_convert_encoding('Aé', 'UCS-2', 'UTF-8'));
// ext/mbstring: '004100e9'
// polyfill:     '4100e900'

Decoding big-endian UCS-2 produced by ext/mbstring or another system gives the wrong characters as well. getEncoding() already maps UTF-16 and UTF-32 to their big-endian names for the same reason. This adds UCS-2 to that list.

The new assertions in testConvertEncoding() cover both directions with the polyfill. They fail without the change on glibc and pass with it. Converting BMP text to UCS-2 and back now gives the same bytes as PHP 8.5's mbstring. ./phpunit tests/Mbstring passes.

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[nicolas-grekas]

Thank you @wleeaf.