How can I get the name of checkbox framework

I am doing a seat reservation project , but i am using a framework for checkbox to doing this. Now I am trying to save the name of each seat into SESSION but error Notice: Undefined index: is occur.

Here is where the code of checkbox I am using: Seat reservation code.

I did try to code like this to get the name of each seat.

HTML:

<form method="POST">
    <div id="1_2" role="checkbox" value="80" aria-checked="false" focusable="true" tabindex="-1" class=" seatNumber" name="A2html" method="POST">2</div>
</form>

PHP:

<?php
    if(isset($_POST["btnCheckout"]))
    {
        $A2php = $_POST['A2html'];
        $_SESSION["A2"] = $A2php;
        print_r($_SESSION["A2"]);
    }
?>