如何将多个MySQLi结果编码成正确的json格式?

if ($result->num_rows > 0) {

     // output data of each row
     while($row = $result->fetch_assoc()) {

$post_data = array(
    'item' => array(
    'ID' => $row["id"],
    'Name' => $row["name"],
    'Category' => $row["category"],
    'Saldo' => $row["saldo"],
    'Editor' => $row["editor"],
    'Edited' => $row["reg_date"]
  )
);
echo json_encode($post_data);
if ($result->num_rows > 0) {

     // output da