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run code in 150+ php & hhvm versions
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<?php var_dump( htmlspecialchars( "\xf4\x8f\xbf\xbf", ENT_COMPAT, 'UTF-8' ) );
Output for 4.3.2 - 7.1.0
string(4) "􏿿"
Output for 4.3.0 - 4.3.1

Process exited with code 139.