$username = $_COOKIE['username']; //What user are we updating ?
$chattext = $_POST['chattext']; //What text did this user submit ?

//Let's check to see if this username is in the table.
$usernamequery = "UPDATE members set chattext = 'chattext{$_POST['chattext']}' WHERE username='$username'";
$username = @mysql_query($usernamequery);
$numusername = mysql_affected_rows($username);//Does this username exist ?

if ($numusername = 0) {//If username doesn't exist add to table.
$adduserquery = "INSERT INTO members (username,chattext) VALUES ('$username', '$chattext');
}