php shopping cart ติดปัญหาครับเกี่ยวกับ สั่งซื้อสินค้าแล้วให้ตัดจากสต็อก

	<h1>สินค้า</h1>
            <?
            $tx = "";
			if($_GET['cat_id'])
			{
			$tx .= "and ( product.cate = '$_GET[cat_id]' )";	
			}
            ?>
           <?
           				include("library/con_db.php");
						mysql_query("SET character_set_results=utf8");
						mysql_query("SET character_set_client=utf8");
						mysql_query("SET character_set_connection=utf8");
						$sql = "SELECT * FROM product left join catalog on (product.cate = catalog.cat_id) where 1 $tx order by product.pid desc";
						$result = mysql_query($sql);
						
          	?>      
                
              <div id="mainright">
              <div id="cart">
             
	<td id="cart" align="center" bgcolor="#eeeeff">&nbsp;
	<script>ajaxLoad('post','cart_ss.php','','cart');</script>

    </td>
    </div>
    </div>
            <?	
			while($p = mysql_fetch_array($result)) 
			{?>  
               
		
	
	 <div id="product">
            	<div id="img">
                <img src="read_image.php?pid=<? echo $p['pid']; ?>" width="150" height="150"></div>
                	<div id="info">
                    		<p>  <p> <h4><? echo $p['product_name']; ?></h4> <P>
						<? echo $p['description']; ?>
				<p>
						ราคา: <? echo $p['price']; ?> บาท <p>
                       <div style="color: red;">  คงเหลือ : <? echo $p['stock'];?> ชิ้น  </div></div>
                          
              <div style="float:right;">
                จำนวน <input type="text" size="3" id="<?php echo "q_{$p['pid']}"; ?>" name="<?php echo "q_{$p['pid']}"; ?>"
value="1" class="tx_qul"> หน่วย <button onclick="addCart(<?php echo $p['pid']; ?>)" class="bt_add_cart">หยิบใส่รถเข็น</button>
                
              </div>
              </div><? 
              }  
			  ?> 
              
             
    </div>

<?
	session_start();

	$dblink = mysql_connect("localhost","root","1234");
	mysql_query("USE shopping_cart;");
	mysql_query("SET NAMES utf8");
	mysql_query("SET character_set_results=utf8");
	mysql_query("SET character_set_client=utf8");
	mysql_query("SET character_set_connection=utf8");
	
		     
	$sid = session_id();

	$pid = "";
	if(isset($_POST['pid'])) {
		$pid = $_POST['pid'];
	}
	$act = "";
	if(isset($_POST['act'])) {
		$act = $_POST['act'];
	}
	
	$sql = "";

	if($act=="add") {
		$q = $_POST['q'];
		
		
		
$sql = <<<SQL
	SELECT product_name, price FROM product
	WHERE pid = $pid;
SQL;
	$result = mysql_query($sql);
	$pname = mysql_result($result,0,0);
	$price = mysql_result($result,0,1);

$sql = <<<SQL
	REPLACE INTO cart VALUES
	('$sid', $pid, '$pname', $price, $q, NOW());
SQL;

	}
	else if($act=="del") {
		
$sql = <<<SQL
	DELETE FROM cart
	WHERE pid = $pid;
SQL;
	}
	else if($act=="clear") {
$sql = <<<SQL
	DELETE FROM cart
	WHERE sid = '$sid';
SQL;
	}

	mysql_query($sql);

$sql = <<<SQL
	SELECT * FROM cart
	WHERE sid = '$sid';
SQL;
	$result = mysql_query($sql);
	//header("content-type:text/; charset=utf8");
	header("content-type:text/plain; charset=utf8");
	
	if(mysql_num_rows($result)==0) {
		echo "ไม่มีสินค้าในรถเข็น";
		mysql_close($dblink);
		exit();
	}

$tb = <<<TABLE
	<table width="98%" border="1" bordercolor="gray" style="border-collapse:collapse;">
	<caption>รายการสินค้าในรถเข็น</caption>
	<tr align="right" bgcolor="powderblue">
		<th align="center">สินค้า</th><th>ราคา</th><th>#</th><th>รวม</th>
	</tr>

TABLE;
	$grand_total = 0;
	while($cart = mysql_fetch_array($result)) {
		$sub_total = $cart['price'] * $cart['quantity'];

$tbody = <<<TBODY
	<tr align="right" valign="top">
	<td align="left">
		<div style="float:left;"> 
 			<!--	
			<button onclick="viewProduct({$cart['pid']})" 
				class="bt_cart">ดู</button>&nbsp;
			-->
			<button onclick="delCart({$cart['pid']})" 
				class="bt_cart">ลบ</button>
		</div>

		{$cart['product_name']}

 	</td>
	<td>{$cart['price']}</td>
	<td>{$cart['quantity']}</td>
	<td>$sub_total</td>
	</tr>
TBODY;
		$tb .= $tbody;

		$grand_total += $sub_total;
	}
	$tb .= "<tr><td align=center>รวมทั้งหมด</td><td align=right colspan=3>$grand_total</td></tr>";
	$tb .= "</table>";

if(!isset($_SESSION['Username'])){
	$a=" <button onclick=alert('กรุณาLogin');window.location='login.php';;>สั่งซื้อ</button>  ";
	} else {
	$a= " <button onclick=location='customer_info.php'>สั่งซื้อ</button> ";
	} 
$bt = <<<BT
	<p align="center">
	<button onclick="clearCart()">ลบทั้งหมด</button>
	&nbsp;&nbsp;&nbsp;&nbsp;	 {$a}
	</p>
BT;
	

	echo $tb . $bt; 
	
		foreach($q as $pid => $q)
	{
		$sql11 = "select * from product where pid = '$pid'";
		$result11 = mysql_query($sql11) or die ("error = $sql11");
		$p = mysql_fetch_array($result11);
		//หาสต็อกคงเหลือ
		$stock = $p['stock'];
		$totalp = $stock - $q;
		$sql12 = "update product set stock = '$totalp' where pid ='$pid' ";
		mysql_query($sql12) or die ("error=$sql12");	
	
	}
	
		

		
	mysql_close($dblink);
?>