PHP计算没有正确加起来

Question

我的页面上显示总价格但是有些东西会导致显示不正确。我不确定是什么导致价格计算错误，但我想知道如何解决这个问题。

这是我的代码：

function cart() {
$total=0;

$output = '';
$output .= '<center>';
$output .= '<table class="menufinal" border=0 width=75%>';
$output .= '<tr>';
$output .= '<th>Remove Item</th>';
$output .= '<th>Item Name</th>';
$output .= '<th>Item Price</th>';
$output .= '<th>Quantity</th>';
$output .= '<th>Line Total</th>';
$output .= '</tr>';

foreach($_SESSION as $name => $value) {
    if ($value>0){
        if (substr($name, 0, 5)=='cart_') {
           $id = substr($name, 5, (strlen($name)-5));
           $get = mysql_query('SELECT id, name, price FROM products WHERE id='.mysql_real_escape_string((int)$id));
           while ($get_row = mysql_fetch_assoc($get)) {
                $sub = $get_row['price']*$value;
                $output .= '<tr>';
                $output .= '<td><a href="cart.php?delete=' .$id.'"><img src="x.png"></a><br></td>';
                $output .= '<td>' . $get_row['name'] . '</td>';
                $output .= '<td>&pound ' . number_format($get_row['price'], 2) . '</td>';
                $output .= '<td><a href="cart.php?remove=' .$id. '"style="text-decoration:none"><strong>- </strong></a>' .$value. '<a href="cart.php?add=' .$id. '"style="text-decoration:none"> +</a></td>';
                $output .= '<td>&pound ' . number_format($sub, 2) . '</td>';
                $output .= '</tr>';
            }
        } 
        if (empty($total)) {

            if (empty($sub)) {
                //do nothing
            } else {
                $total = $sub;
            }
        } else {
            $total += $sub;
        }
        $_SESSION['total'] = $total;
    }
}

$output .= '</table>';

if (empty($total)){
    session_destroy();
    header ('Location: index.php');
    exit;    
}

$output .=  '<br><br><br><br><div id="finalPrice">Total: &pound ' . number_format($total, 2) . '<br></div>';
$output .=  '<br><br><br><p><a href="index.php"><img src="dishes.png" width="240" height="152"></a> <img src="spacer.png" width="110"> <a href="checkout.php"><img src="checkout.png" width="240" height="151"></a>';

echo $output;
}