ต้องการให้มีการตรวจสอบจำนวนสินค้า ถ้าใส่จำนวนสินค้าเกิน Stock ให้แสดงว่าสั่งซื้อเกิน รบกวนช่วยหน่อยนะคะ

<?php include "function.php"; include"template.php"; include"shopping_function.php";
	show_header("ตะกร้าสินค้า");  
	show_menu(); 
	if( empty( $_GET["new"] ) && !isset( $_SESSION["cart"]) ) {
		process_message("ตะกร้าของคุณไม่มีในสินค้า", "search_book.php" );
		show_footer() ; exit ;
	} elseif ( !empty( $_GET["new"] ) && !isset( $_SESSION["cart"] ) ) {
		$_SESSION["cart"] = $_GET["new"] ;
		$_SESSION["quality"] = 1 ;
	} elseif ( !empty( $_GET["new"] ) && isset( $_SESSION["cart"] ) ) {
	   if ( stristr($_SESSION["cart"], $_GET["new"] ) == false ) {
	     $_SESSION["cart"] = $_SESSION["cart"]. "|".$_GET["new"];
		 $_SESSION["quality"] = $_SESSION["quality"]. "|". "1";
	   }
	}
	$arrItems = explode("|",  $_SESSION["cart"] );
	$arrquality = explode("|",  $_SESSION["quality"] ) ;
	$total_quality = 0 ;
	$total_price = "0.00";
	?>
<link href="css/templatemo_style.css" rel="stylesheet" type="text/css" />
 	
 
<form action="shopping_process2.php"method="post">

<table width="697" align="center" cellpadding="0" cellspacing="1">  
    <tr><div id="templatemo_content_right">
     <td colspan="5" align="center"><div class="right1">ตะกร้าสินค้า</div> </td>
    </tr>
    <tr>
      <td width="69" align="center" bgcolor="#666633">เลือก</td>
      <td width="236" align="center" bgcolor="#666633">รายการหนังสือ</td>
      <td width="143" align="center" bgcolor="#666633">ราคา</td>
      <td width="142" align="center" bgcolor="#666633">จำนวนที่สั่งซื้อ</td>
      <td width="155" align="center" bgcolor="#666633">ราคารวม</td>
    </tr>
    <tr>
      <?php 
		$dbcon = connect_db();
		for($i = 0 ; $i < count($arrItems) ; $i++){
			$sql = "SELECT * FROM tbbook WHERE bId = '".$arrItems[$i]."'";
			$resultsql = mysqli_query($dbcon,$sql);
			$book = mysqli_fetch_array($resultsql,MYSQL_ASSOC);
			$sqlpb = "SELECT proDiscount FROM tbpromotiondetail AS pb , tbpromotion AS p
							WHERE pb.proId=p.proId AND bId='".$book["bId"]."' AND expireDate IS NULL";
			$resultpb = mysqli_query($dbcon,$sqlpb);
			$rowspb = mysqli_num_rows($resultpb);
		if($rowspb > 0){
			$rowspb = mysqli_fetch_array($resultpb,MYSQL_ASSOC);
			$sale = calculate_saleprice($book["bPrice"],$rowspb["proDiscount"]);
		}else{
			$sale = $book["bPrice"];
		}
		$total_quality = $total_quality + $arrquality[$i];
		$price = $sale * $arrquality[$i];
		$total_price = $total_price + $price ;
		?>
      
      <td align="center" class="bg"><input type="checkbox" name="chkDel[]"  value="<?php echo $arrItems[$i];?>"></td>
      <td align="center" class="bg"><a href="<?php echo "show_bookdetail.php?bid=".$book["bId"];?>"
      				class="menu"><br />
      <img src="<?php echo'display.php?id='.$book["bId"].
			'&table=tbbook&column=bId';?>" width = "91" height="121" border = "0"/><br/><?php echo $book["bId"];?></a></td>
      <td align="right" class="bg"><?php echo number_format($sale,2);?></td>
      
      <td align="right" class="bg"><?php echo $book["bStock"];?>/<?php  echo $arrquality[$i] ?><input name ="qualty[<?php echo $i;?>]" type = "text" value="<?php echo $arrquality[$i];?>" size="2"></td>
      <td align="right" class="bg"><?php echo number_format($price,2);?></td>
      
    </tr>
    <?php }?>
      <td colspan="3" align="center" class="bg">รวม</td>
      <td align="right" class="bg"><?php echo $total_quality;?></td>
      <td align="right" class="bg"><?php echo number_format($total_price,2);?></td>

  
    <?php $costPost = calculate_costPost($total_price);
			  $costEMS = calculate_costEMS($total_price);
			  	if($_SESSION["shipping"] == "2"){
					$total_price = $total_price+$costEMS;
					$cssEMS = "style=\"color:red\"";
				}else{
					$total_price = $total_price+$costPost;
					$cssPost = "style=\"color:red\"";
				}?>
       <tr> 
       <tr<?php echo $cssPost;?>>
       <td colspan="4" class="bg"><input name="rdoship" type="radio" <?php if ($_SESSION["shipping"] != "2"){
		   echo "checked=\"checked\"";}?> value="1" />พัสดุลงทะเบียน</td>
      <td align="right" class="bg"><?php echo number_format($costPost,2);?></td>
    </tr>
    <tr<?php echo $cssEMS;?>>
      <td colspan="4" class="bg"><input name="rdoship" type="radio" <?php if ($_SESSION["shipping"] == "2"){
		   echo "checked=\"checked\"";}?> value="2" />ไปรษณีย์ด่วนพิเศษ ems</td>
      <td align="right" class="bg"><?php echo number_format($costEMS,2);?></td>
    </tr>
    <tr>
      <td colspan="4" class="bg">ราคาที่ต้องชำระ(รวมราคาสินค้าและค่าจัดส่ง)</td>
      <td align="right" class="bg"><?php echo number_format($total_price,2);?></td>
    </tr>
    <tr>
      <td colspan="5" align="center">
      <input type="button" value="เลือกสินค้าต่อ" onclick="window.location=
      'book_all.php'" />
        <input type="submit" name="btnCalculate" value="คำนวณราคาสินค้าใหม่" />
        <input type="submit" name="btnNext"value="สั่งซื้อสินค้า" />
        <input type="submit" name="btnDelete" value="ลบรายการที่เลือก" /></td>
    </tr></div>
</table>

 <?php }
  function calculate_price(){ //คำนวณราคาสินค้า
	  $arrData = $_POST["qualty"];
	  $_SESSION["quality"] = NULL;
	  for($i=0 ; $i < count($arrData) ; $i++ ){
		  if($i == 0){
			  $_SESSION["quality"] = $arrData[$i];
		  }else{
			  $_SESSION["quality"] = $_SESSION["quality"] ."|".$arrData[$i];
		  }
	  }