通过Javascript验证PHP变量

我希望将一个变量从我的PHP代码传递到我的Javascript代码中，以验证一个数字。我正在创建一个购物车场景，其中我有一个变量"in_stock"，用于指定产品的可用库存量。如果我输入的量大于可用的"in_stock"，我需要抛出一个错误。这就是我目前拥有的：

Java脚本

   <script language="javascript">
  function numCheck() 
  {
        var enteredChar = document.getElementById('add_value').value;
        var stockavailable ="<?php echo $stock?>";
    //To check if a non-numerical value is entered
    if (isNaN(enteredChar)) 
    {
        alert("Not a Number!");
        return false;
    }
    //To check if the input is empty
    if (enteredChar=="")
    {
        alert("Empty!");
        return false;
    }
    //To check if the amount entered is not greater than the amount in stock
 if (enteredChar > stockavailable)
    {
        alert("Not enough in stock");
        return false;
    }
 }  
   </script>

这一切都发生在我的PHP调用数据库并在屏幕上返回值的时候。如下：

PHP

$product_id =(int)$_GET['product_id'];
$username = "potiro";
$password = "pcXZb(kL";
$hostname = "rerun";
//connection to the database
$dbhandle = mysql_connect($hostname, $username, $password)
 or die("Unable to connect to MySQL");

//select a database to work with
$selected = mysql_select_db("poti",$dbhandle)
  or die("Could not select examples");
//execute the SQL query and return records
$result = mysql_query("SELECT * FROM products where product_id=$product_id");
echo '<form name="form1">';
echo '<table class="Grocery-table">'; 
while($row = mysql_fetch_array($result))
{
$stock =$row['in_stock'];
$product_id = $row['product_id'];
  echo "<tr><td><b>Product ID</b></td>"; 
  echo "<td>";  
  echo $product_id;
  echo "</td></tr>";
  echo "<tr><td><b>Product Name</b></td>";
  echo "<td>";
  echo $row['product_name'];
  echo "</td></tr>"; 
   echo "<tr><td><b>Unit Price</b></td>";
  echo "<td>";
  echo $stock;
  echo "</td></tr>"; 
  echo "<tr><td><b>Unit Quantity</b></td>";
  echo "<td>";
  echo $row['unit_quantity'];
  echo "</td></tr>"; 
  echo "<tr><td><b>In Stock</b></td>";
  echo "<td>";
  echo $row['in_stock'];
  echo "</td></tr>";
  echo '<tr><td><b>Add</b></td><td><Input type="text" id="add_value" name="cart"></input></td></tr>';
  echo '<tr><td></td><td><input type="submit" value="Submit" onclick="return numCheck()"></td></tr>';
 }
echo "</table>"; 
echo "</form>";
mysql_close($dbhandle);
?>