Testing for NULL

Hi, I am new... hope I am using this list correctly. I am trying to test for a null value, and if it is null, then to use a default value instead. The login page that I have, I want to show nothing in the username field unless it is passed a username from another PHP page via the header function. However, every time I write: if($username == null){ echo "Username is null"; } It executes the echo, but only after giving me an error. Is there a way to suppress this error or check for an undefined instead? J ** Scanned for Viruses **