How to set a cookie for another domain

Question

You cannot set cookies for another domain. Allowing this would present an enormous security flaw.

You need to get b.com to set the cookie. If a.com redirect the user to b.com/setcookie.php?c=value

The setcookie script could contain the following to set the cookie and redirect to the correct page on b.com

<?php
    setcookie('a', $_GET['c']);
    header("Location: b.com/landingpage.php");
?>

Similar to the top answer, but instead of redirecting to the page and back again which will cause a bad user experience you can set an image on domain A.

<img src="http://www.example.com/cookie.php?val=123" style="display:none;">

And then on domain B that is example.com in cookie.php you'll have the following code:

<?php
    setcookie('a', $_GET['val']);
?>

Hattip to Subin

Probaly you can use Iframe for this. Facebook probably uses this technique. You can read more on this here. Stackoverflow uses similar technique, but with HTML5 local storage, more on this on their blog

Setting cookies for another domain is not possible.

If you want to pass data to another domain, you can encode this into the url.

a.com  ->  b.com/redirect?info=some+info (and set cookie) -> b.com/other+page

In case you have a.my-company.com and b.my-company.com instead of just a.com and b.com you can issue a cookie for .my-company.com domain - it will be accepted and sent to both of the domains.

You can't, at least not directly. That would be a nasty security risk.

While you can specify a Domain attribute, the specification says "The user agent will reject cookies unless the Domain attribute specifies a scope for the cookie that would include the origin server."

Since the origin server is a.com and that does not include b.com, it can't be set.

You would need to get b.com to set the cookie instead. You could do this via (for example) HTTP redirects to b.com and back.