How do I display the number of users currently on my website?

CREATE TABLE users_online (
  timestamp int(15) NOT NULL default '0',
  ip varchar(40) NOT NULL default ',
  PRIMARY KEY (ip)
) TYPE=MyISAM;

// returns number of users currently on the site
function getUsersOnline() {
	# Database-specific information:
	$server = "localhost";
	$database = "db_name";
	$user = "user_name";
	$pw = "password";

	# connect to the database
	$connection = @mysql_connect ($server, $user, $pw) or die ("Error connecting to the database.");

	# select the database
	$db = @mysql_select_db ($database) or die ("Error selecting database.");

	# time limit for users online in seconds - you can increase or decrease this if you want
	# recommended: entries in db older than 1 minute will be removed
	$timeout = 60;

	# get the ip address
	$ip = getenv("REMOTE_ADDR");

	# get the time
	$timestamp = time(); 
	$timelimit = $timestamp - $timeout;

	# delete expired users
	$sql = "DELETE FROM users_online WHERE timestamp < $timelimit";
	$result = @mysql_query ($sql);

	# insert current user
	$sql = "INSERT INTO users_online (timestamp, ip) VALUES ('$timestamp', '$ip')";
	$result = mysql_query ($sql);

	# if ip already exists, update the timestamp
	if (mysql_errno() == 1062) {
		$sql = "UPDATE users_online SET timestamp = '$timestamp' WHERE ip = '$ip'";
		$result = mysql_query ($sql);
	}

	# get number of users online
	$sql = "SELECT COUNT(*) as num_users FROM users_online";
	$result = mysql_query ($sql);
	$row = mysql_fetch_array($result);
	$num_users = $row['num_users'];
	
	if ($num_users == 1)
		return "1 user";
	else
		return $num_users . " users";
}

<? echo getUsersOnline(); ?>