Alright, it might be an easy answer but for mine self, Im just stuck in it.

Here is a small script code:

if(empty($_POST["Username"]) || empty($_POST["password"]))
echo "Fields has not been set";
Now what I want is this:
Add a variable if 1 of those 2 are empty, get the variable echo'd out back on the form page.
The problem is the undefined index.
I'm currently not using global arrays and I was wondering how can I move the variable to the previous page.
However once the page has been refreshed, the variable must not be echo'd out because the user hasn't filled in the form yet "this time".

I hope I make it as much understandable to be understood.