php购物车有关问题

php购物车问题

<?php<br />  include ('book_sc_fns.php');<br />  // The shopping cart needs sessions, so start one<br />  session_start();<br /><br />  @$new = $_GET['new'];<br /><br />  if($new) {<br />    //new item selected<br />    if(!isset($_SESSION['cart'])) {<br />      $_SESSION['cart'] = array();<br />      $_SESSION['items'] = 0;<br />      $_SESSION['total_price'] ='0.00';<br />    }<br /><br />    if(isset($_SESSION['cart'][$new])) {<br />      $_SESSION['cart'][$new]++;<br />    } else {<br />      $_SESSION['cart'][$new] = 1;<br />    }<br /><br />    $_SESSION['total_price'] = calculate_price($_SESSION['cart']);<br />    $_SESSION['items'] = calculate_items($_SESSION['cart']);<br />  }<br /><br />  if(isset($_POST['save'])) {<br />    foreach ($_SESSION['cart'] as $isbn => $qty) {<br />      if($_POST[$isbn] == '0') {<br />        unset($_SESSION['cart'][$isbn]);<br />      } else {<br />        $_SESSION['cart'][$isbn] = $_POST[$isbn];<br />      }<br />    }<br /><br />    $_SESSION['total_price'] = calculate_price($_SESSION['cart']);<br />    $_SESSION['items'] = calculate_items($_SESSION['cart']);<br />  }<br /><br />  do_html_header("Your shopping cart");<br /><br />  if(($_SESSION['cart']) && (array_count_values($_SESSION['cart']))) {<br />    display_cart($_SESSION['cart']);<br />  } else {<br />    echo "<p>There are no items in your cart</p><hr/>";<br />  }<br /><br />  $target = "index.php";<br /><br />  // if we have just added an item to the cart, continue shopping in that category<br />  if($new)   {<br />    $details =  get_book_details($new);<br />    if($details['catid']) {<br />      $target = "show_cat.php?catid=".$details['catid'];<br />    }<br />  }<br />  display_button($target, "continue-shopping", "Continue Shopping");<br /><br />  // use this if SSL is set up<br />  // $path = $_SERVER['PHP_SELF'];<br />  // $server = $_SERVER['SERVER_NAME'];<br />  // $path = str_replace('show_cart.php', '', $path);<br />  // display_button("https://".$server.$path."checkout.php",<br />  //                 "go-to-checkout", "Go To Checkout");<br /><br />  // if no SSL use below code<br />  display_button("checkout.php", "go-to-checkout", "Go To Checkout");<br /><br />  do_html_footer();<br />?><br />

代码中的$_SESSION['cart'][$new]是什么意思，为什么要这样子做？