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run code in 150+ php & hhvm versions
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<?php $int=2 ; var_dump( log(($int^pow(2,ceil(log($int, 2)))-1)+1,2));
Output for 4.3.0 - 5.6.28, hhvm-3.10.0 - 3.12.0, 7.0.0 - 7.1.0
float(2)