cookies problem in PHP and AJAX

I face some problem on my script that I use PHP and jquery to create login system.

First I have PHP page contain form for login. when user click submit I use jquery to send data to server

$.post('server_login.php', {username:username.val(), password:password.val()}, function(data){
    alert(data);
}); 

in server_login.php I have function to doing login user.

if($_POST['username']=='username' && $_POST['password']=='1234'){
    $expire = time() + 60*60*24*30; //1 month expired.
    setcookie("user_id", $_POST['username'], $expire);
    echo true;
}

and jquery alert "1" on my login page.

the problem is when i refresh my website and retieve cookie, it not show me.

print_r($_COOKIE);

anything wrong?