<?php

$a = [[4, 3, 2, 1], [5, 6, 7, 8], [12, 11, 10, 9], [13, 14, 15, 16]];

function f($a){foreach($a as$v)$r[]=$_++&1?$v:array_reverse($v);return array_merge(...$r);}

print_r(f($a));