First off, I'm not to sure on ibase, but assuming its similar in all but actual words to mysql...
if ($submit) will work just as well as if (isset($submit)).
Try this
if ($submit)
{
echo "Test";
}
and see if that works.
If that fails try
if ($_POST['submit'])
{
echo "Test";
}
As if register_globals are off the top one won't work.
If you get Test outputted and it still doesn't work, what error messages, if any, do you get, cause at a glance I can't see anything wrong with the code.