Round-trip conversion between Shift_JIS (CP932) and Unicode

conversion.php

<?php

count_unsafe_chars('cp932');

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

function tohexupper($char) {
    return strtoupper(bin2hex($char));
}

function count_unsafe_chars($enc) {
    $count = 0;

    for ($cp = 0x1000; $cp < 0x10000; ++$cp) {

        $c = chr($cp >> 8).chr($cp & 0xff);

        if (!mb_check_encoding($c, $enc)) {
            continue;
        }

        $ret = roundtrip($c, $enc);

        if ($c !== $ret) {
            echo '[', tohexupper($c), ' ', tohexupper($ret), ']';
            ++$count;
        }
    }

    echo PHP_EOL, PHP_EOL;
    echo $enc, PHP_EOL;
    echo 'count:', $count, PHP_EOL;
}