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<?php $array_list= array("Jan 2016"=>2, "Feb 2016"=>4, "Mar 2016"=>2,"Apr 2016"=>0, "Jan 2017"=>9,"Feb 2017"=>2,"Mar 2017"=>5,"Jan 2018"=>4,"Feb 2018"=>6); foreach(array_keys($array_list) as $key){ $years[] = explode(" ", $key)[1]; } foreach(array_unique($years) as $year){ $res[$year] = array_sum(array_intersect_key(array_values($array_list), array_intersect($years, [$year]))); } var_dump($res);
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