CP936 converted to UTF-8

I recently wrote a crawling script. Most of the content captured is normal, but a small amount of it is garbled

Detect the character encoding and the result is CP936

mb_detect_encoding(<span style="color: #800080;">$str</span>, 'GBK, gb2312, GB18030, ISO-8859-1, ASCII, UTF-8', <span style="color: #0000ff;">true</span>)

Try to convert this encoding, but the result is still garbled

mb_convert_encoding($str, 'UTF-8', 'CP936');
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Finally found out that this can be transcoded

iconv('utf-8', 'latin1', $str);

iconv('utf-8','latin1//IGNORE', $str);