Add Variable to PHP Session Array From Dynamically created HTML element within PHP Echo

Question

I am trying to update a PHP Session array 'shopping cart' when 'add to cart' button is clicked without leaving the page.

I have php written to get several variables from a MySQL database, from which I create an HTML element with php echo. Within that HTML, I am trying to add one of those variables to the PHP Session Array when "Add to Cart" button is clicked. My issue is I can't get the variable passed. I can send it to a JavaScript function, but can't use the JavaScript to add it to the Session array.
Any tips on how I can add $isbn to the session array? Is there a different way of doing this that would be better? I would prefer to stay on the same page & not change anything else except updating the Session 'cart'.

`

        //create section for each book result
        if($result->num_rows>0){
            //get variables from db
            while($row=$result->fetch_assoc()){
                $title = $row['title'];
                $author = $row['author'];
                $publisher = $row['publisher'];
                $isbn = $row['isbn'];
                $price = $row['price'];

                echo "
                    <tr>
                        <td align='left'>
                        <!-- on button click add to cart session array -->
                            <button name='btnCart' 
                                id='btnCart' 
                                type='submit'
                                onClick='cart(\"{$isbn}\")'>Add to Cart
                            </button>
                        </td>
                        <td rowspan='2' align='left'>{$title}</br>
                            By {$author}</br>
                            <b>Publisher:</b> {$publisher}</br>
                            <b>ISBN:</b> {$isbn} <br> 
                            <b>Price: $</b> {$price}
                        </td>
                    </tr>
                    <tr>
                        <td align='left'>
                            <button name='review' 
                                id='review' 
                                onClick='review(\"{$isbn}\", \"{$title}\")'>Reviews
                            </button>
                        </td>
                    </tr>
                    <tr>
                        <td colspan='2'>
                            <p>_______________________________________________</p>
                        </td>
                    </tr>
                ";
            }
        }

`

Thanks!