Online PHP editor | refs for 68XoB

<?php $array1 = [ '2012' => ['1' => 13, '2' => 22, '3' => 18, '5' => 19], '2011' => ['1' => '4', '3' => 13], ]; $array2 = [ '2012' => ['1' => 44, '4' => 34, '5' => 38, '6' => 29], '2011' => ['1' => 9, '2' => 11, '3' => 1], ]; $array3 = [ '2012' => ['2' => 5, '4' => 3, '6' => 1, '8' => 11], '2011' => ['3' => 9, '4' => 14, '9' => 9], ]; $defaults = array_fill_keys(range(1, 12), 0); $result = []; foreach ([$array1, $array2, $array3] as $array) { foreach ($array as $year => $row) { $result[$year] ??= $defaults; foreach ($row as $m => $v) { $result[$year][$m] += $v; } } } var_export($result);