shopping cart ERROR!!! - คือ ทำระบบซื้อขายสินค้าออนไลน์ พอคลิ๊กหยิบใส่ตระกร้า

<?
session_start();
$id_prd=$_GET[id_prd]; 
$_SESSION['sess_id']=session_id;
$_SESSION['sess_name']='$sess_name';
$_SESSION['sess_photo']='$sess_photo';
$_SESSION['sess_price']='$sess_price';
$_SESSION['sess_num']='$sess_num';
if(count ($_SESSION['sess_id'])=="0"){
$check=1;
}else if (!in_array($id_prd,$_SESSION['sess_id'])){
$check=1;
}
if($check==1){
include "connect.php" ;
$sql="select * from tb_product where id_prd='$id_prd' ";
$result=mysql_db_query($dbname,$sql);
$rs=mysql_fetch_array($result);

$sess_id[]=$rs[id_prd];
$sess_name[]=$rs[name_prd];
$sess_photo=$rs[photo_prd];
$sess_price[]=$rs[price_prd];
$sess_num[]=1;
}
header("Location:cart.php");
?>

<?
session_start(); 
if(count ($_SESSION['sess_id'])==0){
echo"<p align=center>ยังไม่มีสินค้าอยู่ในตระกร้า</p>";
}else{

for ($i=0;$i<count($_SESSION['sess_id']);$i++){
$total_unit=$sess_num[$i]*$sess_price[$i];
$total=$total+$total_unit; 
echo"<tr>
<td align='center' bgcolor='#FFF4F4'><input type='checkbox' name='prd_del[]' value='$sess_id[$i]'></td>
<td align='center' bgcolor='#FFF4F4'>$sess_photo[$i]</td>
<td align='center' bgcolor='#FFF4F4'>$sess_name[$i]</td>
<td align='center' bgcolor='#FFF4F4'><input type='text' name='prd_num[]' 
value='$sess_num[$i]' size='2'></td>
<td align='center' bgcolor='#FFF4F4'>$sess_price[$i]</td>
<td align='center' bgcolor='#FFF4F4'>$total_unit</td>
</tr>";
}
?>