mb_strtoupper converts non-alphabetic characters in Shift_JIS (CP932)

Description

mb_strtoupper converts non-alphabetic characters in Shift_JIS (CP932)

var_dump(
    "\x81\xE0" === mb_strtoupper("\x87\x90", 'cp932')
);

\x81\xE0 and \x87\x90 mean ≒ (U+2252: Approximately Equal to or the Image Of).

var_dump(
    "\u{2252}" === mb_convert_encoding("\x81\xE0", 'utf-8', 'cp932'),
    "\u{2252}" === mb_convert_encoding("\x87\x90", 'utf-8', 'cp932')
);

\x87\x90 is the one of NEC special characters and was registered in duplicate for historical reasons.

The unintended conversion is caused by a round-trip conversion between Unicode and Shift_JIS.

var_dump(
    "\x81\xE0" === roundtrip("\x87\x90", 'cp932'),
    "\x81\xE0" === roundtrip("\x81\xE0", 'cp932')
);

function roundtrip($char, $enc) {
    return mb_convert_encoding(mb_convert_encoding($char, 'utf-8', $enc), $enc, 'utf-8');
}

As far as I know, 398 characters in Shift_JIS are affected by round-trip conversion. The test code is here.

The same problem applies to mb_scrub, mb_strtolower, mb_convert_case.

var_dump(
    "\x81\xE0" === mb_scrub("\x87\x90", 'cp932'),
    "\x81\xE0" === mb_strtolower("\x87\x90", 'cp932'),
    "\x81\xE0" === mb_strtoupper("\x87\x90", 'cp932'),
    "\x81\xE0" === mb_convert_case("\x87\x90", MB_CASE_TITLE, 'cp932')
);

PHP Version

PHP 8.2.10

Operating System

Debian 11.7 (Google ChromeOS 117.0)

Reproduce the supplied CP932 examples and use the mb_scrub, mb_strtolower, mb_strtoupper, and mb_convert_case entry points as the starting scope. Done means the duplicate NEC character sequence remains distinct from its canonical CP932 sequence instead of being changed by round-trip conversion, with the affected behavior covered by tests.