ถามเรื่อง select max หน่อยครับ อยากให้ฟิวรันเองอัตโนมัติ

<?
	include('conn_mysql.php');
	
	$product_id = $_POST["product_id"];
	$dates = date("Y-m-d");
	$customer_id = $_POST["customer_id"];
	$sql = "insert into tborder (customer_id,pricetotal,dates) VALUES ('$customer_id','44','$dates')";
		$query = mysql_db_query($dbname,$sql)or die(mysql_error());
		
	for($i=0;$i<count($product_id);$i++)
	{		
		$num = "num".$product_id[$i];
		$num = $_POST["$num"];
		$total = "total".$product_id[$i];
		$total = $_POST["$total"];
		$product_code = $_POST["$product_code"];
		 
	//อยู่บรรทัดนี้อ่าครับ

		$sql2 = "insert into order_details (order_no,product_id,product_code,quantity) VALUES ('1','$product_id[$i]','$product_code[$i]','$num')";
		$query = mysql_db_query($dbname,$sql2)or die(mysql_error());
		
		echo "<script>alert('บันทึกข้อมูลเรียบร้อย');</script>";
		}
		echo "<script>window.location='ordercustomer.php';</script>";
	mysql_close();   
?>

<?php
	$sql="select max(order_no) from order_details";
	$result=mysql_query($sql);
	$r=mysql_fetch_array($result);
	$id_max=$r[0]+1;
echo str_pad($id_max, 3, "0", STR_PAD_LEFT);
?>