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<?php $selectedCats = [2,4,6]; array_walk($selectedCats, function (&$cat) { return $cat * 2; }); var_dump($selectedCats);
Output for 5.6.0 - 5.6.30, hhvm-3.18.5 - 3.22.0, 7.0.0 - 7.3.0rc3
array(3) { [0]=> int(2) [1]=> int(4) [2]=> int(6) }