3v4l.org

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<?php var_dump( $conv = mb_convert_encoding( $bin = hex2bin("ffb383"), "UTF-8", "GB18030" ), mb_check_encoding($bin, "GB18030"), bin2hex(mb_convert_encoding($conv, "GB18030", "UTF-8")) );
Output for 8.1.0 - 8.1.30, 8.2.0 - 8.2.24, 8.3.0 - 8.3.12
string(4) "?硟" bool(false) string(6) "3fb383"
Output for 7.4.0 - 7.4.33, 8.0.1 - 8.0.30
string(5) "ÿ硟" bool(false) string(12) "81308b37b383"

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