Get current User

Hi, I am trying to get the current user ID so they can change their password in a content management system. My login code is as follows;

$errorMessage = '';
if (isset($_POST['txtUserId']) && isset($_POST['txtPassword'])) {
	include 'library/config.php';
	include 'library/opendb.php';
	$userId   = $_POST['txtUserId'];
	$password = $_POST['txtPassword'];
	// check if the user id and password combination exist in database
	$sql = "SELECT user_id 
	        FROM adminuser
			WHERE user_id = '$userId' AND user_password = md5('$password')";
	$result = mysql_query($sql) or die('Query failed. ' . mysql_error()); 
	if (mysql_num_rows($result) == 1) {
		// the user id and password match, 
		// set the session
		$_SESSION['db_is_logged_in'] = true;
		// after login we move to the main page
		header('Location: index.php');
	} else {
		$errorMessage = 'Sorry, wrong user id / password';
	include 'library/closedb.php';

I’m then using this to check they are logged in to access pages;

// like i said, we must never forget to start the session

// is the one accessing this page logged in or not?
if (!isset($_SESSION['db_is_logged_in']) || $_SESSION['db_is_logged_in'] !== true) 

	// not logged in, move to login page
	header('Location: login.php');


Can anyone help? I’ve tried using $_SESSION[‘user_id’], and lots of other variations. Any help would be great!