Pages refresh on submission on RedHat Server - POST problem

Its a rather strange thing. I tested all my pages on my Win2000-Apache configuration and uploaded them to my clients RedHat server. There, on submission, all pages just refresh themselves. All my forms are set to submit to the same page and I check in the beginning for the submit button variable. If its set, the form data is processed - else its assumed that its the first time that the page is being viewed. I loaded the pages on another RedHat server and all pages work perfectly there. I experimented a bit and found that if I change the action of forms to GET, they work. But with passwords and file uploads, I cant do that for all pages. Have tried $HTTP_POST_VARS also - doesnt work. Any idea of what could be the problem? If the submit button is named "submit_login", I am checking as follows: if ($submit_login <> "") to see if the form has been submitted. Hve tried isset also. Thanks, Arijit