array(1) { ["month"]=> array(2) {
["January"]=> array(4) { [0]=> array(1) { [1]=> string(4) "2100" } [1]=> array(1) { [2]=> string(2) "50" } [2]=> array(1) { [3]=> string(4) "7858" } [3]=> array(1) { [4]=> string(4) "3535" } }
["December"]=> array(2) { [0]=> array(1) { [1]=> string(3) "123" } [1]=> array(1) { [2]=> string(4) "1900" } } } }

string(111) "{"month":{"January":[{"1":"2100"},{"2":"50"},{"3":"7858"},{"4":"3535"}],"December":[{"1":"123"},{"2":"1900"}]}}"

array_keys($array['month']);