Ayuda..con mi carrito de compras.please...=(

Hola "davidcortesb" Primero,deseo agradecerte que me dediques un poco de tu valioso tiempo. Espero que me puedas ayudar o aunque sea dar una idea de como solucionar este problemita:
me sale este error en mi carrito de compras
Notice: Undefined index: cart in C:\wamp\www\cart-demo\cart-demo\inc\functions.inc.php on line 3
You have no items in your shopping cart
estos 3 estan en la carpeta inc
este es la pagina functions.inc.php
<?php
function writeShoppingCart() {
 $cart = $_SESSION['cart'];
 if (!$cart) {
  return '
<p>You have no items in your shopping cart</p>
';
 } else {
  // Parse the cart session variable
  $items = explode(',',$cart);
  $s = (count($items) > 1) ? 's':'';
  return '
<p>You have <a href="cart.php">'.count($items).' item'.$s.' in your shopping cart</a></p>
';
 }
}
function showCart() {
 global $db;
 $cart = $_SESSION['cart'];
 if ($cart) {
  $items = explode(',',$cart);
  $contents = array();
  foreach ($items as $item) {
   $contents[$item] = (isset($contents[$item])) ? $contents[$item] + 1 : 1;
  }
  $output[] = '<form action="cart.php?action=update" method="post" id="cart">';
  $output[] = '
<table>
';
  foreach ($contents as $id=>$qty) {
   $sql = 'SELECT * FROM books WHERE id = '.$id;
   $result = $db->query($sql);
   $row = $result->fetch();
   extract($row);
   $output[] = '
<tr>
';
   $output[] = '
<td><a href="cart.php?action=delete&id='.$id.'" class="r">Remove</a></td>
';
   $output[] = '
<td>'.$title.' by '.$author.'</td>
';
   $output[] = '
<td>£'.$price.'</td>
';
   $output[] = '
<td><input type="text" name="qty'.$id.'" value="'.$qty.'" size="3" maxlength="3" /></td>
';
   $output[] = '
<td>£'.($price * $qty).'</td>
';
   $total += $price * $qty;
   $output[] = '
</tr>
';
  }
  $output[] = '
</table>
';
  $output[] = '
<p>Grand total: <strong>£'.$total.'</strong></p>
';
  $output[] = '
<div><button type="submit">Update cart</button></div>
';
  $output[] = '</form>';
 } else {
  $output[] = '
<p>You shopping cart is empty.</p>
';
 }
 return join('',$output);
}
?>
mysql.class.php
<?php
/**
* MySQL Database Connection Class
* @access public
* @package SPLIB
*/
class MySQL {
    /**
    * MySQL server hostname
    * @access private
    * @var string
    */
    var $host;
    /**
    * MySQL username
    * @access private
    * @var string
    */
    var $dbUser;
    /**
    * MySQL user's password
    * @access private
    * @var string
    */
    var $dbPass;
    /**
    * Name of database to use
    * @access private
    * @var string
    */
    var $dbName;
    /**
    * MySQL Resource link identifier stored here
    * @access private
    * @var string
    */
    var $dbConn;
    /**
    * Stores error messages for connection errors
    * @access private
    * @var string
    */
    var $connectError;
    /**
    * MySQL constructor
    * @param string host (MySQL server hostname)
    * @param string dbUser (MySQL User Name)
    * @param string dbPass (MySQL User Password)
    * @param string dbName (Database to select)
    * @access public
    */
    function MySQL ($host,$dbUser,$dbPass,$dbName) {
        $this->host=$host;
        $this->dbUser=$dbUser;
        $this->dbPass=$dbPass;
        $this->dbName=$dbName;
        $this->connectToDb();
    }
    /**
    * Establishes connection to MySQL and selects a database
    * @return void
    * @access private
    */
    function connectToDb () {
        // Make connection to MySQL server
        if (!$this->dbConn = @mysql_connect($this->host,
                                      $this->dbUser,
                                      $this->dbPass)) {
            trigger_error('Could not connect to server');
            $this->connectError=true;
        // Select database
        } else if ( dbName,$this->dbConn">!@mysql_select_db($this->dbName,$this->dbConn) ) {
            trigger_error('Could not select database');
            $this->connectError=true;
        }
    }
    /**
    * Checks for MySQL errors
    * @return boolean
    * @access public
    */
    function isError () {
        if ( $this->connectError )
            return true;
        $error=mysql_error ($this->dbConn);
        if ( empty ($error)...