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<?php $array = [ '7777777777', '7777777777', '6666666666', '6666666666', '2222222222', ]; $grouped = [[], []]; foreach ($array as $i => $v) { $grouped[(int) $i & 1][] = $v; } $grouped[1] = array_reverse($grouped[1]); var_export( array_merge(...$grouped) );

preferences:
19.13 ms | 406 KiB | 5 Q