warning ved session

hej
jeg har en kode der skal give en bruger mulighed fro at logge ind, men der er et problem med session. Der kommer følgende fejlmeddelelse :

Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at c:\apache\apache\htdocs\registrering\loginform.php:1) in c:\apache\apache\htdocs\registrering\loginform.php on line 5



og koden er som følger :


<?
ob_start();
?>
<?php
session_start();
require("sidehoved.html");
if (isset($HTTP_POST_VARS[userid]) && isset($HTTP_POST_VARS[pass]))

{

  // if the user has just tried to log in
  $userid = $HTTP_POST_VARS['userid'];
  $pass = $HTTP_POST_VARS['pass'];

  $db_conn = mysql_connect("localhost", "root")  or die(mysql_error());
  mysql_select_db('registrering', $db_conn)  or die(mysql_error());
  $query = "select * from kunde where brugernavn='$userid' and pass='$pass'";

$result = mysql_query($query, $db_conn) or die(mysql_error());
 
  if (mysql_num_rows($result) >0 )
  {
    // if they are in the database register the user id
    $HTTP__SESSION_VARS['valid_user']  = $userid; 
  }
}
?>



<html>
<body>
<!--<h1>Home page</h1> -->
<?
if (isset($HTTP_SESSION_VARS['valid_user']))
  {
      header("Location: http://127.0.0.1/registrering/til10.php");               
      exit;
//    echo 'You are logged in as: '.$HTTP_SESSION_VARS['valid_user'].' <br />';
//    echo '<a href="logout.php">Log out</a><br />';
  }
  else
  {
    if (isset($userid))
    {
      // if they've tried and failed to log in
      echo 'Du har skrevet forkert brugernavn og password !';
    }
    else
    {
      // they have not tried to log in yet or have logged out
      echo 'You are not logged in.<br />';
    }
  }
    // provide form to log in
?>




    <center>
    <form method="post" action="loginform.php">
    <table>
    <tr><td>Userid:</td>
    <td><input type="text" name="userid"></td></tr>
    <tr><td>Password:</td>
    <td><input type="PASSWORD" name="pass"></td></tr>
    <tr><td colspan="2" align="center">
    <input type="submit" value="Log in"></td></tr>
    </table></form>
<br>
<br>
<br>
<br>
<br>

<?

require("sidefod.html");

ob_end_flush();
?>