Because browser security there are heavy limitations if you load external content into a window. External content is content who comes from another domain (URL) then the WinLIKE page comes from. Note: all the following servernames point to same physical computer, but are recognized by browser as different machines (domains): localhost, myserver, myserver.mydomain.com, 127.0.01, 192.168.1.1

Note: P3P browsers as IE6 block cookies from external sources if you don't allow this:
PHP: header("p3p: CP=\"CAO PSA OUR\"");
ASP: Response.AddHeader "P3P","CP=""CAO PSA OUR"""
Otherwise your session variables won't work because the server doesn't recognize the client.