用两个外键将数据插入到一个表中

我有两个表(用于POS系统)名为:purchase和product。purchase表的列有:poid (PK, auto_increment)， prodid (FK)， poquantity。product表的列有:prodid (PK, auto_increment)， prodname, price, quantity。我想把数据插入到purchase表中。

表单代码如下:

<?php
  $tbl_name="product"; 
$con = mysql_connect("localhost","root","");
if (!$con)
 {
 die('Could not connect: ' . mysql_error());
 }
mysql_select_db("pos", $con);
$prodid=$row['prodid'];
$result = mysql_query("SELECT prodname FROM product where prodid = '$prodid'");
 echo "<form action='addprodcon.php?id=$row[prodid]' method='POST'>
  <input name='add' type='submit' value='ADD'></br></br>
  </form>";
  ?>

下面是我创建的插入代码:

<?php
include('includes/dbcon.php');
$prodid = $_GET['prodid'];
$sql1="SELECT prodid FROM product WHERE prodid='$prodid'";
if(mysql_query($sql1))
{
$sql2="INSERT INTO purchase (`prodid`, `poquantity`) 
VALUES
('$prodid','$_POST[poquantity]')";
 }
if (mysql_query($sql2))
{
    // Success
 if ( $poquantity > $_POST['quantity'])
  {
  echo "You already reached the maximum quantity";
  }
}
else 
{
    die('Error on query 2');
}

?>

每当我点击提交按钮，它就会显示: unknown index: prodid请提供帮助或建议?我还是一个学习PHP的学生。谢谢!:)