...
let form = new FormData();
form.append('id', '1');
let data = {"name":"John", "sex":"male", "age":"33"};
form.append('data', data);
...
var_dump($_POST["id"]);
// string(1) "1"
var_dump($_POST["data"]);
// string(15) "[object Object]"
var_dump($_POST["data"]["sex"]);
// string(1) "["
var_dump($_POST["data"] -> sex);
// NULL NULL
let data = {"name":"John", "sex":"male", "age":"33"};
form.append('data', JSON.stringify(data));
$data = json_decode($_POST['data']);