Online PHP editor | refs for 67YBS

<?php $array = [ "2022-10-30" => ['▶'], "2022-10-31" => ['▶'], "2022-11-09" => ['▶'], "2022-11-07" => ['▶'] ]; $result = []; foreach ($array as $key => $value) { $ym = substr($key, 0, 7); if (!isset($lookup[$ym])) { $result[$key] = $value; $lookup[$ym] = $key; } elseif ($lookup[$ym] < $key) { unset($result[$lookup[$ym]]); $result[$key] = $value; $lookup[$ym] = $key; } } var_export($result);