how to test if a session is already made

[ronjon]

'tis me again...your beloved newbie.

this time i wanna know how to test if a session is already made.
i.e...

<?
session_start();
header("Cache-control: private"); //IE 6 Fix

if (!$_SESSION['username'])  <<<this is the part that i dont knowhow to do...but you get the idea of what im trying to do
{
	$username = 0;
	$authenticated = 0;
}

else
{
	$username = $_SESSION['username'];
	$authenticated = $_SESSION['authenticated'];
}

echo $username;
echo $authenticated;
?>

can ya help me out?

[mrhappiness]

session_start();
$_SESSION['authenticated'] = isset($_SESSION['authenticated']) ? $_SESSION['authenticated'] : false;

if ($_SESSION['authenticated'])
  echo $_SESSION['username'];
else
  echo 'not authenticated';

something like this?

[ronjon]

isset... thats what i was looking for. thankyou very much sir.