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)

Reproduzca los ejemplos de CP932 proporcionados y utilice los puntos de entrada mb_scrub, mb_strtolower, mb_strtoupper y mb_convert_case como alcance inicial. Se considera terminado cuando la secuencia de caracteres NEC duplicada sigue siendo distinta de su secuencia CP932 canónica, en lugar de ser modificada por una conversión de ida y vuelta, y el comportamiento afectado está cubierto por pruebas.