如何按类别，项目和费率的格式安排会话数组数据？

Question

my data now stored in json in the format stated below which is actually a separate array for category, item and rate. But I want it to store per category chosen..If category 1 chosen then it must show how many items under that category and rate for each item.

Currently my JSON order as below:

{"level":{"Primary":"1","Upper Secondary":"2"},"sub":{"2":"bm","1":"maths"},"rate2":{"bm":"10","bi":"20","maths":"30"},"submit":"submit"}

$_SESSION['info'][$level]=array('level'=>$level,array('subject'=>$subject,'rate'=>$rate));

How do I arrange this correctly in SESSION array as below order:

Primary
  [bm=10
  ,bi=20]

Upper Secondary
  [maths=30]

And how do I retrieve it later after decoding as below in php file to be stored inside database please?

$return = $_POST;

$return["json"] = json_encode($return);

form input:

<ul class="small-block-grid-3 medium-block-grid-3 large-block-grid-2">
    <li><input type="checkbox" name="level[Primary]" id="level" class="level" value="1"><label>Primary</label></li>
    <li><input type="checkbox" name="level[Upper Secondary]" id="level" class="level" value="2"><label>Upper Secondary</label></li>
    <li><input type="checkbox" name="level[University]" id="level" class="level" value="3"><label>University</label></li>
    <li><input type="checkbox" name="level[Lower Secondary]" id="level" class="level" value="4"><label>Lower Secondary</label></li>
    <li><input type="checkbox" name="level[Pre University]" id="level" class="level" value="5"><label>Pre University</label></li>
    <li><input type="checkbox" name="level[Skills/Languages]" id="level" class="level" value="6"><label>Skills/Languages</label></li>                 
</ul>

//below is inside while loop as data fetched from database using ajax//

echo '<table><tr><td>'. $subjects . '</td><td><input type="checkbox" name="sub['.$levels_id.']" id="sub" value="'.$subjects.'"></td>';
echo'<td><input type="textbox" name="rate2['.$subjects.']" class="rate2" value="" id="'.$subjects.'"></td></tr></table>';