Session problem

Hi
I use this code for my accesscontrol file:
accesscontrol.php:

<?php
session_start();
include_once 'db.php';
include_once 'common.php';
if(isset($_POST['username']))
{
    $username = $_POST['username'];
}
else if(isset($_SESSION['username']))
{
    $username = $_SESSION['username'];
    }
if(isset($_POST['pass']))
{
    $pass = $_POST['pass'];
}
else if(isset($_SESSION['pass']))
{
    $pass = $_SESSION['pass'];
}

if(!isset($username)) {
  ?>
  <!DOCTYPE html PUBLIC "-//W3C/DTD XHTML 1.0 Transitional//EN"
    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
  <html xmlns="http://www.w3.org/1999/xhtml">
  <head>
    <title> Please Log In for Access </title>
    <meta http-equiv="Content-Type"
      content="text/html; charset=utf-8" />
  </head>
  <body>
  <h1> Login Required </h1>
  <p>You must log in to access this area of the site. If you are
     not a registered user, <a href="signup.php">click here</a>
     to sign up for instant access!</p>
  <p><form method="post" action="<?=$_SERVER['PHP_SELF']?>">
    User ID: <input type="text" name="username" size="8" /><br />
    Password: <input type="password" name="pass" SIZE="8" /><br />
    <input type="submit" value="Log in" />
  </form></p>
  </body>
  </html>
  <?php
  exit;
}

$_SESSION['username'] = $_POST['username'];
$_SESSION['pass'] = $_POST['pass'];

$username = $_POST['username'];
$pass = $_POST['pass'];
dbConnect("articles");
$sql = "SELECT * FROM user WHERE username = '$username' AND pass ='$pass'";
$result = mysql_query($sql);
if (!$result) {
  error('A database error occurred while checking your '.
        'login details.\\nIf this error persists, please '.
        'contact you@example.com.');
}

if (mysql_num_rows($result) == 0) {
  unset($_SESSION['username']);
  unset($_SESSION['pass']);
  ?>
  <!DOCTYPE html PUBLIC "-//W3C/DTD XHTML 1.0 Transitional//EN"
    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
  <html xmlns="http://www.w3.org/1999/xhtml">
  <head>
    <title> Access Denied </title>
    <meta http-equiv="Content-Type"
      content="text/html; charset=utf-8" />
  </head>
  <body>
  <h1> Access Denied </h1>
  <p>Your user ID or password is incorrect, or you are not a
     registered user on this site. To try logging in again, click
     <a href="<?=$_SERVER['PHP_SELF']?>">here</a>. To register for instant
     access, click <a href="signup.php">here</a>.</p>
  </body>
  </html>
  <?php
  exit;
}
$username = mysql_result($result,0,'username');
?>

PHP:

Then I wanted to test the session I had set up. I coded these 4 simple pages:
pro1.php:

<?php
include('accesscontrol.php');
?>
this is pro1<br>
<a href=pro2.php>pro2</a>

PHP:

pro2.php:

<?php
include('accesscontrol.php');
?>
this is pro2<br>
<a href=prono.php>prono</a>

PHP:

prono.php:

this is prono<br>
<a href=pro3.php>pro3</a>

PHP:

pro3.php

<?php
include('accesscontrol.php');
?>
pro3

PHP:

pro means a protected page and prono mean available to everyone. I expected users to be logged in after filling 1 login form, and remain logged in after they visit pro2, prono, and want to view pro3 ( a protected page again) but i was not successful. Please help me where I'm wrong?

the current result of the code is:
pro1:
first it is the login form, then the page is shown upon login
After I click on the "pro2" link in pro1.php, this error occures:

Notice: Undefined index: username in g:\programs(2)\easyphp1-8\www\ha\accesscontrol.php on line 48

Notice: Undefined index: pass in g:\programs(2)\easyphp1-8\www\ha\accesscontrol.php on line 49

Notice: Undefined index: username in g:\programs(2)\easyphp1-8\www\ha\accesscontrol.php on line 51

Notice: Undefined index: pass in g:\programs(2)\easyphp1-8\www\ha\accesscontrol.php on line 52
this is pro2
prono
then, I click on prono and it's ok, ofcourse:
this is prono
pro3
Then I click on the pro3 link and I'm redirected to the login form. how do i fix it?
thanks

 

This is not errors it's just notices... to turn off notices add this code line to top of your scripts:

<?php
error_reporting(E_ALL ^ E_NOTICE);

//other code
?>

PHP:

 

Thanks.
Now no error appears. but how do i keek users logged in when they want to view pro3 and they have logged in for viewing pro1 and skipped a protected page by visiting a available-to-everyone page then returning to a protected one without closing their browser?