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run code in 150+ php & hhvm versions
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<?php $a = array(1, 2, 3, 4); function transformMe(&$val) { $val *= 2; } array_walk($a, 'transformMe'); var_dump($a);
based on edIIg
Output for 4.3.0 - 5.6.28, hhvm-3.10.0 - 3.12.0, 7.0.0 - 7.1.0
array(4) { [0]=> int(2) [1]=> int(4) [2]=> int(6) [3]=> int(8) }