如何从 Ajax 调用修改 Cookie

我有这个代码:

window.onload = function() {
        document.cookie = 'foo=bar; expires=Sun, 01 Jan 2012 00:00:00 +0100; path=/';
        var xhr = new XMLHttpRequest();
        xhr.open("GET", "/showcookie.php",true);
        xhr.setRequestHeader("Cookie", "foo=quux");

        xhr.setRequestHeader("Foo", "Bar");
        xhr.setRequestHeader("Foo", "Baz");

        xhr.withCredentials = true;
        var pre = document.getElementById('output');
        xhr.onreadystatechange = function() {
            if (4 == xhr.readyState) {
                pre.innerHTML += xhr.responseText + "
";
            }
        };
        xhr.send(null);
    };

还有这个/showcookie.php

<?php

print_r($_COOKIE);

?>

Array
(
    [Host] => localhost
    [User-Agent] => 
    [Accept] => 
    [Accept-Language] => pl,en-us;q=0.7,en;q=0.3
    [Accept-Encoding] => gzip,deflate
    [Accept-Charset] => ISO-8859-2,utf-8;q=0.7,*;q=0.7
    [Keep-Alive] => 115
    [Connection] => keep-alive
    [foo] => Baz
    [Referer] =>
    [Cookie] => foo=bar
)

Array
(
    [foo] => bar
)