I wrote an article on this issue a while back, Cross Domain AJAX.
The easiest way to handle this if you have control of the responding server is to add a response header for:
Access-Control-Allow-Origin: *
This will allow cross-domain Ajax. In PHP, you'll want to modify the response like so:
<?php header('Access-Control-Allow-Origin: *'); ?>
You can just put the setting in the Apache configuration or htaccess file.Header set Access-Control-Allow-Origin *
It should be noted that this effectively disables CORS protection, which very likely exposes your users to attack. If you don't know that you specifically need to use a wildcard, you should not use it, and instead you should whitelist your specific domain:
<?php header('Access-Control-Allow-Origin: http://example.com') ?>