<?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 ]; $totals = []; foreach ($array as $date => $amount) { $year = substr($date, -4); $totals[$year] = ($totals[$year] ?? 0) + $amount; } var_export($totals);
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