PHP cookie issue on redirect

PHP
1

I’m not exactly sure what the problem is but the cookies aren’t available for subdomains. Is there any way around this, or is there a better way to set the cookies domain? Thanks!

<?php
ini_set("session.cookie_domain", ".example.com");

// Set cookie and redirect when user change city
if( isset($_POST['city']) && $_POST['city'] != '' ){
    $cookie_expire = time() + 50400;
    setcookie('city', $_POST['city'], $cookie_expire, '/');

    header("Location: http://".$_POST["city"].".example.com");
    die();
}

// Redirect if user selected default city
if (isset($_COOKIE["city"])) {
    $subdomain = array_shift(explode(".",$_SERVER['HTTP_HOST']));

    if ($_COOKIE["city"] != $subdomain) {
        header("Location: http://".$_COOKIE["city"].".example.com");
        die();
    }
}