User Authentication Session Handling Problems - Authorization server variables not staying across pages

<?php
session_start();
session_register('auth');

require_once('database.inc');

// These $d_<something> variables will be placed in the template
$d_html_head = 'Some portal DART';
$d_header = 'The header - DART';
$d_status = NULL;
$d_content = NULL;
$d_nav = '<h2>Link set 1</h2><ul><li><a href="#">Link 1</a></li><li><a href="#">Link 2</a></li><li><a href="#">Link 3</a></li></ul><h2>Link set 2</h2><ul><li><a href="#">Link 4</a></li><li><a href="#">Link 5</a></li><li><a href="#">Link 6</a></li></ul><h2>Link set 3</h2><ul><li><a href="#">Link 7</a></li><li><a href="#">Link 8</a></li><li><a href="#">Link 9</a></li></ul>';
$d_footer = 'copyright info';


$q = '';

// Database handling part
$dartdb = new dbhandler;
$connection = $dartdb->setconnection( 'dbadmin', 'dbpassword', 'localhost');
if(!$connection)
  $d_status .= "Unable to get a connection <BR /> $dartdb->errorstring <BR />";
$connection = $dartdb->setdatabase('dartdb');
if(!$connection)
  $d_status .= "Unable to select DART database <BR /> $dartdb->errorstring <BR />";


if ( isset($_GET['q']) )
  $q = $_GET['q'];
if ( $q == 'login')
  {
  // Check the 'user' and 'pass' against database and set  
  // 'auth' based on the result
  $loginmessage = "The Employee number or the password given is wrong.  Please try again.";
  $_SERVER['auth'] = 'NO';
  
  $user = NULL;
  $pass = NULL;  
  $user = $_POST['user'];
  $pass = $_POST['pass'];
  
  
  $query = "SELECT * FROM dart_emp WHERE empid = '".$user."'";
  $dartdb->query($query);
  if ( $user != NULL && $dartdb->result != NULL )
    {
    $array = $dartdb->fetch_object();
    if( isset($array->empid)
        && $array->empid == $user
        && $array->password == $pass )
      {
      $loginmessage = "Login successful.";
      $_SERVER['auth'] = 'YES';
      }
    }
  $d_status .= $loginmessage;
  }
else if ($q == 'logout')
  {
  // User has logged out. Hence set the 'auth' to 'NO'
  $_SERVER['auth'] = 'NO';
  $d_status .= 'Logged out. <BR />';
  }

if( isset($_SERVER['auth']) && $_SERVER['auth'] == 'YES' )
  {
  $d_status .= 'Authorized access <BR />';
  $d_content .= 'Content, content. <BR />Logout <A href="?q=logout">link</A>.';
  }
else
  {
  //Show the login form
  if ($q != 'logout')
    $d_status .= 'Not logged in. <BR />';
  $d_content .= '<form action="?q=login" method="post" name="login">
  Employee Number: <input type="text" name="user" size="6" maxlength="6" id="user" /> <BR />
  Password: <input type="password" name="pass" size="30" maxlength="30" id="pass" /> <BR />
  <input type="submit" name="login" value="Login" id="login" />
  </form>';
  }

// This is the templating system I use.  The above $d_<something> values
// are replaced in the appropriate places
require 'template/page.tpl';
?>

session_register('username');
$_SESSION['username'] = $username;
session_register('password');
$_SESSION['password'] = $password;

session_start();
if(isset($_SESSION['username']) && isset($_SESSION['password'])){header("location:login_successful.php?error=loggedin");}

session_start();
session_destroy();

function session_delete($sname = 'Current User') {
    # Set Session Name to a Variable
    $name = session_name();
    /* Empty the Cookie from Session */
    if (!headers_sent() ) { setcookie($name,"",0,"/"); }
    /* Remove the Cookie Value */
    unset($_COOKIE[$name]);
    /* Remove all the Info from the Super Global */
    $_SESSION = array();
    /* Free all session variables */
    session_unset();
    /* Destroy all data registered to a session */
    if (session_destroy() === FALSE) {
        return FALSE;
    } else {
        return TRUE;
    }    
}

<?php
// Use of session_register() is deprecated
$barney = "A big purple dinosaur.";
session_register("barney");

// Use of $_SESSION is preferred, as of PHP 4.1.0
$_SESSION["zim"] = "An invader from another planet.";

// The old way was to use $HTTP_SESSION_VARS
$HTTP_SESSION_VARS["spongebob"] = "He's got square pants.";
?>