พี่ๆช่วยดูโค้ดให้หน่อยครับ เกี่ยวกับการหยิบสินค้าเกินสต็อค Shopping_cart ครับ

<?

	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 = "";
	

	$sql9 = "select * from product where pid ='$pid' ";
	$result9 = mysql_query($sql9) or die ("error=$sql9"); 
	$b = mysql_fetch_array($result9);
		
	/*$q = $_POST['q']; 
	if($q <= $b['stock'])   {
		*/
	if($act=="add") {
		$q = $_POST['q'];   
		if($q > $b['stock']) {  echo"<script>alert('เกินจำนวนสินค้า');window.history.back();</script>"; }
                                               
             
		else{
		
$sql = <<<SQL
	SELECT product_name, price,stock FROM product
	WHERE pid = $pid;
SQL;
	$result = mysql_query($sql);
	$pname = mysql_result($result,0,0);
	$price = mysql_result($result,0,1);
	$stock = mysql_result($result,0,2);
	

$sql = <<<SQL
	REPLACE INTO cart VALUES
	('$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="gray">
		<th align="center"><font color="white">สินค้า</th><th><font color="white">ราคา</th><th><font color="white">#</th><th><font color="white">รวม</th></font>
	</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; 
	
	
	
	mysql_close($dblink);
?>