Updating text field when change the value of a select field

In this test I have a simple form with two fields. I need upadate the text field when a user select an option in the select field.

This is the html code:

<form id="form1" name="form1" method="post" action="">
  <input type="text" name="dst" id="dst" value="" />
  <select name="contactos" id="contactos">
    <option value="-1">No selected</option>
    <option value="1">Option 1</option>
    <option value="2">Option 2</option>
  </select>
</form>

This is the jquery code:

<script>
$(document).ready(function() {
   $("#contactos").change(function(event){
   var id = $("#contactos").find(':selected').val();
     $.getJSON('contacback.php?contactoID='+id, function(data) {
         $.each(data, function(i,item) {
           if (item.field == "dst") {
              $("#dst").val(item.value); }
           });
        });
    });
});
</script>

(added)This is the php code:

<?php require_once('Connections/cnx3.php'); ?>
<?php

$colname_rsConta开发者_如何学运维ctos = "-1";
if (isset($_GET['contactoID'])) {
$colname_rsContactos = $_GET['contactoID'];
}

mysql_select_db($database_cnx3, $cnx3);
$query_rsContactos = sprintf("SELECT contactoID, telefono FROM contactos WHERE contactoID = %s",
GetSQLValueString($colname_rsContactos, "int"));
$rsContactos = mysql_query($query_rsContactos, $cnx3) or die(mysql_error());

while ($fila = mysql_fetch_array($rsContactos)) {

$json = array('field' => 'dst',
              'value' => $fila['telefono']);
}

echo json_encode($json);

mysql_free_result($rsContactos);
?>

The query to php is sent OK and the JSON answer it's also OK image of JSON reply

But the value of #dst is not updated. Any helps is welcome !!

---

(Answered in a question edit and comments. Converted to a community wiki answer. See Question with no answers, but issue solved in the comments (or extended in chat) )

The OP wrote:

Solved!! Thanks to @Attack the jQuery script:

$("#contactos").change(function(event){
    var id = $("#contactos").find(':selected').val();
      $.getJSON('contacback.php?contactoID='+id, function(data) {
        if (data.field == "dst") {
            $("#dst").val(data.value);
            }
      });
 });