How to get ID on a specific row in PHP and MySQL
<p>PHP is giving me error of </p>

<blockquote>
  <p>Notice: Undefined index: product_id in C:\xampp\htdocs\odieinventory\admin\add_sales.php on line 6</p>
</blockquote>

<p>This is my query</p>

<pre><code>&lt;?php 
  if (isset($_POST['addCart'])) {
    $product_id = $_POST['product_id'];
    $qtyBuy = $_POST['qtyBuy'];

    $addQuery = "INSERT INTO sales (product_id, quantity) VALUES ($product_id, $qtyBuy)";
    $execQuery = mysqli_query($connection, $addQuery);
  }
?&gt;
</code></pre>

<p>This is my table</p>

<pre><code>&lt;form action="add_sales.php" method="POST"&gt;
        &lt;?php 

          $query = "SELECT * FROM products";
          $exec = mysqli_query($connection, $query);

          while ($row = mysqli_fetch_array($exec)) {
            $product_id = $row['product_id'];
            $product_name = $row['product_name'];
            $description = $row['description'];
            $product_quantity = $row['quantity'];
            $product_price = $row['sell_price'];

         ?&gt;
            &lt;tr&gt;
                &lt;td class="text-center"&gt;&lt;?php echo $product_id; ?&gt;
                  &lt;input type="hidden" name="product_id" value="&lt;?php echo $product_id; ?&gt;"&gt;
                &lt;/td&gt;
                &lt;td&gt;&lt;?php echo $product_name; ?&gt;&lt;/td&gt;
                &lt;td&gt;&lt;?php echo $description; ?&gt;&lt;/td&gt;
                &lt;td&gt;&lt;?php echo $product_price; ?&gt;&lt;/td&gt;
                &lt;td&gt;&lt;?php echo $product_quantity; ?&gt;&lt;/td&gt;

                &lt;td&gt;&lt;input type="number" min="1" max="999" name="qtyBuy"&gt;&lt;/td&gt;
            &lt;/tr&gt;
         &lt;?php } ?&gt;

        &lt;/tbody&gt;
      &lt;/table&gt;
      &lt;/div&gt;
      &lt;div class="form-group"&gt;

          &lt;input type="submit" name="addCart" value="Add Items to Cart" class="btn btn-info pull-right"&gt;

      &lt;/div&gt;
   &lt;/div&gt;
&lt;/form&gt;
</code></pre>

<p><img src="https://i.stack.imgur.com/5E65v.png" alt="My Order Page"></p>

<p>I would also like to ask for advice on how to insert records on my cart. By clicking 'Add Items to Cart', I like to pass ONLY the items with values on the quantitiy input on the cart. How to achieve this with PHP?</p>