$ _GET变量取决于重定向URL

I have the following website

http://localhost/showmecow/dashboard/

And inside I have a PHP script running from an Iframe. The script is located at

http://localhost/showmecow/phpscript

I try to connect to FB, when I put the redirect URL as http://localhost/showmecow/phpscript it works fine, FB redirects back to this script with $_GET filled with code field and state field but now this script is shown on its own and not in the Iframe.

when I put the redirect URL as http://localhost/showmecow/dashboard/ it does not work, FB redirects back to the dashboard with empty $_GET and when I call getSessionFromRedirect it fails in isValidRedirect.

Any ideas on why it may happen?

Thanks

For Future reference, As I was saying in the comment, _GET was filled for the upper level which is the dashboard but not for the bottom level which is the PHP script in the Iframe. What I did is to save these parameters in the DB and extract them in the script.

I prefer that method because I have a membership DB which means each user gets a space.

If I didn't have it, I would have found a way to pass it to the script using command line argument.