<?php function swapOutermost(&$a, $i = 0) { $last = count($a) - 1 - $i; if ($i < $last) { [$a[$i], $a[$last]] = [$a[$last], $a[$i]]; swapOutermost($a, ++$i); } } $array = [1, 2, 3, 4, 5, 6, 7]; swapOutermost($array); var_export($array);
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