php - Warning: Cannot modify header information - headers already sent by

Hi All,

I keep having a warning message appear when I try login. Can you help me rectify the problem.

Warning: Cannot modify header information - headers already sent by (output started at /path/to/login.php:15) in /path/to/login.php on line 231

here's the php code before the html:

        //allow sessions to be passed so we can see if the user is logged in

        session_start();
        //connect to the database so we can check, edit, or insert data to our users table
        $con = mysql_connect('XXXXXXXXXXXXXXX', 'XXXXXXXXXXXX', 'XXXXXXXXXX') or die(mysql_error());
        $db = mysql_select_db('XXXXXXXXXXXXXXXXX', $con) or die(mysql_error());
        //include out functions file giving us access to the protect() function made earlier
        include "functions.php";
?>

Here is the functions.php file:

function protect($string){
    $string = trim(strip_tags(addslashes($string)));
    return $string;
}

?>

and finally here is the PHP inside the HTML:

        //If the user has submitted the form
        if($_POST['submit']){
            //protect the posted value then store them to variables
            $username = protect($_POST['username']);
            $password = protect($_POST['password']);


            //Check if the username or password boxes were not filled in
            if(!$username || !$password){
                //if not display an error message
                echo "You need to fill in a Username and a Password!";
            }else{
                //if the were continue checking

                //select all rows from the table where the username matches the one entered by the user
                $res = mysql_query("SELECT * FROM `users` WHERE `username` = '".$username."'");

                $num = mysql_num_rows($res);

                //check if there was not a match
                if($num == 0){
                    //if not display an error message
                    echo "The Username you supplied does not exist!";
                }else{
                    //if there was a match continue checking

                    //select all rows where the username and password match the ones submitted by the user

                    $res = mysql_query("SELECT * FROM `users` WHERE `username` = '".$username."' AND `password` = '".$password."'");
                    $num = mysql_num_rows($res);

                    //check if there was not a match
                    if($num == 0){
                        //if not display error message
                        echo "The Password you supplied does not match the one for that username!";
                    }else{
                        //if there was continue checking


                        //split all fields fom the correct row into an associative array
                        $row = mysql_fetch_assoc($res);

                        //check to see if the user has not activated their account yet
                        if($row['active'] != 1){
                            //if not display error message
                            echo "You have not yet Activated your account!";
                        }else{
                            //if they have log them in


                            //set the login session storing there id - we use this to see if they are logged in or not
                            $_SESSION['uid'] = $row['id'];
                            //show message
                            echo "You have successfully logged in!";

                            //update the online field to 50 seconds into the future
                            $time = date('U')+50;
                            mysql_query("UPDATE `users` SET `online` = '".$time."' WHERE `id` = '".$_SESSION['uid']."'");

                            //redirect them to the usersonline page

                            header('Location: usersOnline.php');
                        }
                    }
                }
            }
        }   

?>

The main issue arises when I try redirect them to my usersOnline.php page.

//redirect them to the usersonline page
                            header('Location: usersOnline.php');

I have minimised all white space, but still have problems. Where the error informs me, line 15 is my meta id's (Description & Keywords - SEO). Does this pose problems in PHP?

Any help given is much appreciated as I am relatively new with PHP.

Thanks,
Darren