Online PHP editor | refs for t9k6a

<?php $array = [ "Jan 2016" => 2, "Feb 2016" => 4, "Mar 2016" => 2, "Apr 2016" => 0, "Jan 2017" => 9, "Feb 2017" => 2, "Mar 2017" => 5, "Jan 2018" => 4, "Feb 2018" => 6 ]; foreach ($array as $date => &$amount) { $year = substr($date, -4); $amount = ['month' => $amount, 'total' => &$ref[$year]]; $ref[$year] = ($ref[$year] ?? 0) + $amount['month']; } var_export($array);