Well i saw you know and you´re already using the superglobals array e.g.: $_GET, $_POST, $_REQUEST ...
So what i wanted to say was:
If you are using the get method in your form, try using post method instead. And of course, in your php code use $_GET or $_POST depending on it.
And in last case to use the same $_GET or $_POST in your php code, instead of $HTTP_GET_VARS or $HTTP_POST_VARS!