php set subdomain cookies and redirect

[gmax]

I have the cookies and subdomain selection for header:

<script type="text/javascript" src="/static/js/jquery-1.3.2.min.js"></script>
<script type="text/javascript">
$(function(){
var city = readCookie('city');
if(city !=null && city !=''){
window.location.href = 'http://' + city + '.example.com';
}
$('#citygo').change(function(){
var city = $(this).val();
window.location.href = 'http://' + city + '.example.com';
});
});
function createCookie(name,value,days) {
if (days) {
    var date = new Date();
    date.setTime(date.getTime()+(days*24*60*60*1000));
    var expires = "; expires="+date.toGMTString();
}
else var expires = "";
document.cookie = name+"="+value+expires+"; path=/";
}
function readCookie(name) {
var nameEQ = name + "=";
var ca = document.cookie.split(';');
for(var i=0;i < ca.length;i++) {
    var c = ca[i];
    while (c.charAt(0)==' ') c = c.substring(1,c.length);
    if (c.indexOf(nameEQ) == 0) return c.substring(nameEQ.length,c.length);
}
return null;
}
function eraseCookie(name) {
createCookie(name,"",-1);
}
</script>
<select id="citygo">
<option value="0">Select City</option>
<option value="amsterdam">Amsterdam</option>
<option value="newyork">New York</option>
<option value="london">London</option>
<option value="cardiff">Cardiff</option>
</select>

Now I need to work on the server side to set cookies to remember and redirect to a visited subdomain. The code below is not working but should be something like that. Would someone show me how to set cookie? Any help will be very much appreciated.

<?php 
if (isset($_COOKIE["city"])) { 
if ($_COOKIE["city"] == 'city') { 
header("window.location.href = 'http://' + city + '.example.com'"); 
} 
} 
?>