Ajax和php重定向

This is what I try to do:

1. server A display a page
2. javascript in this page call a server A php script with ajax
3. server A script check right and other data
4. if all is ok, server A redirect to resource on server B

Problem :

I cannot redirect my php answer to ajax, probably due to cross domain protection.

What can I do?

I don't want use file_get_contents because the resource on server B can be quite heavy so I don't want download it twice (once by serverA, second by client)

just get a response from server which consists of js code to redirect you to the required URL

for example

echo "<script> window.location=url_to_redirect;</script> ";