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

http://php.about.com/od/finishedphp1/ss/php_login_code_4.htm

i'm just putting this code in. didn't modify it at all.

line 51-57:

Warning: Cannot modify header information - headers already sent by on line 54
Warning: Cannot modify header information - headers already sent by  on line 54
Warning: Cannot modify header information - headers already sent by on line 56

full code below:

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<?php // Connects to your Database
mysql_connect("your.hostaddress.com", "username", "password") or die(mysql_error()); mysql_select_db("Database_Name") or die(mysql_error());

//Checks if there is a login cookie

if(isset($_COOKIE['ID_my_site']))

//if there is, it logs you in and directes you to the members page

{     $username = $_COOKIE['ID_my_site'];     $pass = $_COOKIE['Key_my_site'];          $check = mysql_query("SELECT * FROM users WHERE username = '$username'")or die(mysql_error());     while($info = mysql_fetch_array( $check ))              {         if ($pass != $info['password'])             {                          }         else             {             header("Location: members.php");             }         } }

//if the login form is submitted

if (isset($_POST['submit'])) {

// if form has been submitted
// makes sure they filled it in
    if(!$_POST['username'] | !$_POST['pass']) {
        die('You did not fill in a required field.');
    }
    // checks it against the database

    if (!get_magic_quotes_gpc()) {
        $_POST['email'] = addslashes($_POST['email']);
    }
    $check = mysql_query("SELECT * FROM users WHERE username = '".$_POST['username']."'")or die(mysql_error());

//Gives error if user dosen't exist
$check2 = mysql_num_rows($check);
if ($check2 == 0) {
        die('That user does not exist in our database. <a href=add.php>Click Here to Register</a>');
                }
while($info = mysql_fetch_array( $check ))     
{
$_POST['pass'] = stripslashes($_POST['pass']);
    $info['password'] = stripslashes($info['password']);
    $_POST['pass'] = md5($_POST['pass']);

//gives error if the password is wrong
    if ($_POST['pass'] != $info['password']) {
        die('Incorrect password, please try again.');
    }
else { // if login is ok then we add a cookie      $_POST['username'] = stripslashes($_POST['username']);      $hour = time() + 3600; setcookie(ID_my_site, $_POST['username'], $hour); setcookie(Key_my_site, $_POST['pass'], $hour);     //then redirect them to the members area header("Location: members.php"); } } } else {     // if they are not logged in ?> <form action="<?php echo $_SERVER['PHP_SELF']?>" method="post"> <table border="0"> <tr><td colspan=2><h1>Login</h1></td></tr> <tr><td>Username:</td><td> <input type="text" name="username" maxlength="40"> </td></tr> <tr><td>Password:</td><td> <input type="password" name="pass" maxlength="50"> </td></tr> <tr><td colspan="2" align="right"> <input type="submit" name="submit" value="Login"> </td></tr> </table> </form> <?php } ?>