$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.
$usernamequery = "SELECT username FROM members WHERE username='$username';
$username = @mysql_query($usernamequery);
$numusername = mysql_num_rows($username);//Does this username exist ?

if ($numusername = 0) {//If username doesn't exist add to table.
$adduserquery = "INSERT INTO members (username,chattext) VALUES ('$username', '$chattext');
}
elseif ($numusername = 1) {//If the username already exists update chattext.
$updatechat = "UPDATE members SET chattext='chattext{$_POST['chattext']}'"
}