combobox encadeadas

boa tarde,

alguem pode-me dizer o que está mal aqui ? é que eu não percebo de jquery e o um amigo meu me enviou uns exemplos para ver como funcionava as combobox mas elas não aparecem.

o código é o seguinte:

index.php

Código :

<?php
$mysqli = new mysqli("localhost", "root", "", "jquery");

if (mysqli_connect_errno()) {
printf("Connect failed: %s\n", mysqli_connect_error());
exit();
}
?>
<html>
<head>
<meta charset="ISO-8859-1">
<script src="https://code.jquery.com/jquery-latest.min.js"></script>
<script>
$(document).ready(function(){
$.get("get_countries.php", function(data, status) {
data = $.parseJSON(data);
if(data) {
$.each(data, function(index, element) {
var option = $('<option />');
option.attr('value', element.countries_id).text(element.country);
$('#comboCountries').append(option);
});
$('#comboCountries').fadeIn();
}
});

$('#comboCountries').change(function(e) {
//se o item selecionado for o 0 não faz nada
if($(this)[0].selectedIndex!=0) {
var countries_id = $(this).val();
//limpar a comboRegions
$('#comboRegions').find('option').remove().end();
$.get("get_regions.php",
{
countries_id: countries_id

},
function(data, status) {
data = $.parseJSON(data);

if(data!='0') {
$.each(data, function(index, element) {
var option = $('<option />');
option.attr('value', element.regions_id).text(element.regions);
$('#comboRegions').append(option);
});
$('#comboRegions').fadeIn();
$('#comboRegions').trigger('change');
}
else {
$('#comboCitys').hide();
$('#comboRegions').hide();
}
});

}
else {
//se não tivermos nada selecionado
$('#comboCitys').fadeOut();
$('#comboRegions').fadeOut();
}
});

$('#comboRegions').change(function(e) {
var regions_id = $(this).val();
$('#comboCitys').find('option').remove().end();
$.get("get_citys.php",
{
regions_id: regions_id
},
function(data, status) {
data = $.parseJSON(data);
if(data!='0') {
$.each(data, function(index, element) {
var option = $('<option />');
option.attr('value', element.citys_id).text(element.citys);
$('#comboCitys').append(option);
});
$('#comboCitys').fadeIn();
}
else {
$('#comboCitys').hide();
}
});
});

});
</script>
</head>
<body>
<h1>Comboboxs encadeadas</h1>
<div id="myForm">
<form method="POST" name="firstForm">
<select id="comboCountries" style="display: none;">
<option value="0">Indique a país desejado</option>
</select>

<select id="comboRegions" style="display: none;"></select>
<select id="comboCitys" style="display: none;"></select>

</form>
</div>
<div id="msg"></div>
</body>
</html>

get_countries.php

Código :

<?php
$mysqli = new mysqli("localhost", "root", "", "jquery");

if (mysqli_connect_errno()) {
printf("Connect failed: %s\n", mysqli_connect_error());
exit();
}

$countries = array();

if($stmt = $mysqli->prepare("Select * from countries order by country")) {
$stmt->execute();
$stmt->bind_result($countries_id, $country, $population, $flag, $is_active);

while ($stmt->fetch()) {

$array = [
'countries_id' => $countries_id,
'country' => $country,
'population' => $population,
'flag' => $flag,
'is_active' => $is_active
];

array_push($countries,$array);
}

$stmt->close();
}

if (count($countries)==0) die("0");
else die(json_encode($countries));
?>

get_regions.php

Código :

<?php
$mysqli = new mysqli("localhost", "root", "", "jquery");
$mysqli->set_charset('utf8');

if (mysqli_connect_errno()) {
printf("Connect failed: %s\n", mysqli_connect_error());
exit();
}

//echo "id: $_GET[countries_id]";

$regionsArray = array();

if($stmt = $mysqli->prepare("Select * from regions where is_active=1 and countries_id=?")) {
$stmt->bind_param('i', $_GET['countries_id']);
$stmt->execute();
$stmt->bind_result($regions_id, $regions, $countries_id, $is_active);

while($stmt->fetch()) {

$array = [
'regions_id' => $regions_id,
'regions' => $regions,
'countries_id' => $countries_id,
'is_active' => $is_active
];
array_push($regionsArray, $array);
}

$stmt->close();
}
if (count($regionsArray)==0) die("0");
else die(json_encode($regionsArray));
?>

get_citys.php

Código :

<?php
$mysqli = new mysqli("localhost", "root", "", "jquery");
$mysqli->set_charset('utf8');

if (mysqli_connect_errno()) {
printf("Connect failed: %s\n", mysqli_connect_error());
exit();
}

$citysArray = array();

if($stmt = $mysqli->prepare("Select * from citys where is_active=1 and regions_id=?")) {
$stmt->bind_param('i', $_GET['regions_id']);
$stmt->execute();
$stmt->bind_result($citys_id, $citys, $regions_id, $is_active);

while($stmt->fetch()) {

$array = [
'citys_id' => $citys_id,
'citys' => $citys,
'regions_id' => $regions_id,
'is_active' => $is_active
];
array_push($citysArray, $array);
}

$stmt->close();
}
if (count($citysArray)==0) die("0");
else die(json_encode($citysArray));
?>

ficaria muito agradecido se alguem me ajudasse aqui sff