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run code in 150+ php & hhvm versions
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<?php mb_substitute_character(0xfffd); var_dump(bin2hex(mb_convert_encoding("A\xfd\xbf\xbf\xbf\xbf\xbfB", 'UTF-16', 'UTF-8')));
Output for 5.4.0 - 7.1.0
string(32) "0041fffdfffdfffdfffdfffdfffd0042"
Output for 5.3.0 - 5.3.29
string(8) "00410042"
Output for 4.3.0 - 5.2.17
string(12) "0041fffd0042"