3v4l.org

run code in 150+ php & hhvm versions
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<?php var_dump( bin2hex( mb_convert_encoding( "\xf4\x90\xbf\xbf", 'UTF-16BE', 'UTF-8' ) ) );
Output for 5.4.20 - 5.6.28, hhvm-3.10.0 - 3.12.0, 7.0.0 - 7.1.0
string(16) "003f003f003f003f"
Output for 5.4.0 - 5.4.19
string(12) "003f003f003f"
Output for 4.3.0 - 5.3.29
string(8) "dc03dfff"