如何在wordpress网站上setcookie？

I have problem with seting and reading cookie on wordpress site.

Here is what I'm doing:

1.Sending over js id to php

ajax.js

jQuery.post("/wp-content/themes/mytheme/ajax.php", {post_id: post_id}, function(data){

});

ajax.php

setcookie('myids', $_COOKIE['myids'].$_POST['post_id'], time()+3600*24*100, '/');

2.When I read that cookie ( echo $_COOKIE['myids']; )in /wp-content/themes/mytheme/index.php

I get value which I set and that is ok but when i try to read that cookie in /wp-content/themes/mytheme/ajax.php I am not getting cookie.

What I am doing wrong?

Solution: put ajax.php in root ( /ajax.php ) and then it works.