Created a subdomain and now my POST data doesn't work

Question

My php site with its main domain name works fine. I decided to add a subdomain (mobile) to support mobile devices. I created the subdomain and copied all my php files into the new folder 'mobile'.

I can access the new subdomain using the mobile.mainDomain.com and the files seem to work from a static point of view (no 404's, no error 500 server errors, etc.). All the files are there and display. However, when I try to login (on the index.php page) I enter in my user name but when index.php calls the index_dispatcher.php file to process the login, my index.php form field frm_user_name is empty.

For debugging I put the line

print $frm_user_name;

in the index_dispatcher.php file and it prints nothing (empty).

So there must be something else that needs to be set for the POST data to work with a subdomain?