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run code in 200+ php & hhvm versions
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<?php $days = array('2'); function alter_days(&$day) { return 'bla'; } var_dump(array_filter($days, 'alter_days'));
Output for 4.3.0 - 5.6.30, hhvm-3.11.1 - 3.17.3, 7.0.0 - 7.1.7
array(1) { [0]=> string(1) "2" }
Output for hhvm-3.10.1

Process exited with code 137.