How to properly write code in order to send a php muliple array to JSON?

I need to perform a task which consists in sending a php multiple array to JSON, but it doesn't work and i don't know if i am missing something!

Since it is 2 days that i am having this trouble i decided asking here, because i haven't found nothing much on how properly write the code, especially for the php part.

How can i improve this code in order to make it work? Is the php part done correctly? What function should i use, if there are, to make everything going right?

AJAX call:

    function ajaxcall(ID)
    {
        $.ajax({
          type: 'POST',
          url: 'visualize.php',
          data: {id: ID},
          dataType: 'json',
          success: function(data) {
            for (i=0; i<data.length; i++)
            {
              var state = data.i.state;
              var comment = data.i.comment;
              var date1 = data.i.date1;
              var date2 = data.i.date2;
              $('.comments').append("<b> state: </b> " + state + "<br>"
                                + "<b> comment: </b> " + comment + " <br>"
                                + "<b> date1: </b> " + date1 + " <br>"
                                + "<b> date2: </b> " + date2 + " <br>");
            }
          },
          error : function(err, req) {
            alert("SOMEWHING WENT BAD");
          }
      });
    }

visualize.php

    $id = $_POST['id'];
    $result = mysql_query("SELECT * FROM Cstate WHERE cod = $id ORDER BY Cod2 ASC");
    $num = mysql_num_rows($result);
    $i=0;
    while($i < $num)
    {
      $arr2 = array("$i" => array('state' => mysql_result($result, $i, "StateC"),
                        'comment' => mysql_result($result, $i, "Comment"),
                        'date1' =>  mysql_result($result, $i, "Date1");,
                        'date2' => mysql_result($result, $i, "Date2");));
      $i++;
    }
    echo json_encode($arr2);

EDIT: if i print the "arr2" in my browser it works as a proper JSON string.