SJIS-win→SJIS

php 
<?php
$str = "あアｱ①㈱㌦〜" . pack('C*', 0xF8, 0x9F);
echo mb_convert_encoding($str, 'SJIS-win', 'SJIS-win') . "\n";
echo mb_convert_encoding($str, 'SJIS', 'SJIS') . "\n";
echo mb_convert_encoding($str, 'SJIS', 'SJIS-win') . "\n";
echo mb_convert_encoding($str, 'SJIS-win', 'SJIS') . "\n";

あアｱ①㈱㌦〜[絵文字]
あアｱ①㈱㌦〜?
あアｱ???〜?
あアｱ①㈱㌦〜?

あれ？