Simply put, if you try to read the value of a get variable, what happens if said variable had not be put into the URL. Example: you request the page test.php, in that file it tries to read the value of $_GET['message']. What happens in this case? dose the value just get returned as ''?

Dose this mean, that if I am always expecting a value to be entered, and am not willing to accept a value of '' that I can just do something like

$foo = $_GET['bar'];
if($foo == ''){
  // Handle my 'error'
}
else
{
  // $foo should now have a value that I can work with
}

Please bare in mind I know that I could use isset($_GET['bar']) But I don't just want to know if it is set, I don't care if it is or not, I just care if it has a value that is more then just an empty string.