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<?php $arr1 = [ "Jan 01, 2013" => ["COM" => 100, "RES" => 200, "FOO" => 5], "Oct 28, 2014" => ["COM" => 300, "RES" => 400] ]; $arr2 = [ "Jan 01, 2013" => ["COM" => 10, "RES" => 20], "Oct 28, 2014" => ["COM" => 30, "RES" => 40] ]; var_export( array_map( fn($set) => array_map( fn($v) => array_sum((array) $v), $set ), array_merge_recursive($arr1, $arr2) ) );

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