$jsonString = '{
                "years": {
                "2016": [1, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0],
                "2017": [0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0],
                "2018": [0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0],
                "2019": [0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0],
                "2020": [0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0],
                "2008": [0, 0, 0, 0, 0, 0, 0, 1, 7, 6, 4, 6],
                "2009": [1, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
                "2010": [0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
                "2011": [0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
                "2013": [1, 0, 0, 3, 3, 0, 0, 0, 0, 0, 0, 0],
                "2014": [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1],
                "2015": [0, 0, 0, 0, 0, 0, 0, 1, 1, 0, 0, 0]
            }
        }';

        // array dönüştürülüyor
        $arr = json_decode($jsonString, true);
        $total = 0;
        foreach ($arr["years"] as $item) {
            $total += array_sum($item);
        }
        print_r($total);