Set cookie on multiple domains with PHP or JavaScript

Question 1

Do what Google is doing. Yes, Google is doing this same trick to login the user to YouTube and other Google services which are on different domains.

Create a PHP file that sets the cookie on all 3 domains. Then on the domain where the theme is going to set, create a HTML file that would load the PHP file that sets cookie on the other 2 domains. Example:

<html>
   <head></head>
   <body>
      <p>Please wait.....</p>
      <img src="http://domain2.com/setcookie.php?theme=whateveryourthemehere" />
      <img src="http://domain3.com/setcookie.php?theme=whateveryourthemehere" />
   </body>
</html>

Then add an onload callback on body tag. The document will only load when the images completely load that is when cookies are set on the other 2 domains. Onload Callback :

<head>
   <script>
   function loadComplete(){
      window.location="http://domain1.com";//URL of domain1
   }
   </script>
</head>
<body onload="loadComplete()">

setcookie.php

We set the cookies on the other domains using a PHP file like this :

<?php
if(isset($_GET['theme'])){
   setcookie("theme", $_GET['theme'], time()+3600);
}
?>

Now cookies are set on the three domains.

Source - My Blog

Question 2

You could include the other two domains in iFrames. For more information search for "third party cookies". However depending on the browser settings, the user might not accept third party cookies. There are some other ideas here: What's your favorite cross domain cookie sharing approach?