Cubic calculation SESSION PHP - php

Question

I am trying to do a cubic calculator program where the calculated cubic meter is always added to the result of the previous calculation and displayed. I'm trying to do it with SESSION but it doesn't work out. When I start the calculation it does not show the value. How can I store the previous calculation and add it to the new one?
<?php
session_start();
include 'settings.php';
$title = 'Főoldal';
require_once 'components/htmltop.php';
require_once 'components/navbar.php';
if(isset($_POST["add_to_cart"]))
{
if(isset($_SESSION["cubic_cart"]))
{
$item_array = array(
'item_width' => $_POST["width"],
'item_length' => $_POST["length"],
'item_thickness' => $_POST["thickness"]
);
$_SESSION["cubic_cart"][0] = $item_array;
}
}
?>
<div class="col container text-center">
<div class="row">
<div class="col-sm">
<div class="container">
<form action="" method="post" class="col mx-auto mb-2 form-group p-3 text-center">
<div class="form-group col-md">
<input id="textBar" type="text" name="width" value="" class="i1 form-control mb-3 mx-auto" autocomplete="off" required>
</div>
<div class="row">
<div class="form-group col-md-6">
<select type="text" name="length" value="" class="s2 form-control mb-3 mx-auto" required>
<option selected>Length</option>
<option value="2.5">2.5</option>
<option value="2">2</option>
<option value="3">3</option>
</select>
</div>
<div class="form-group col-md-6">
<select type="text" name="thickness" value="" class="s2 form-control mb-3 mx-auto" required>
<option selected>Thickness</option>
<option value="0.029">29</option>
<option value="0.031">31</option>
</select>
</div>
</div>
<button type="submit" name="add_to_cart" class="btn btn-lg btn-success" onclick="return confirm('Biztos rögzíteni szeretné?')">Calculate</button>
</form>
</div>
<div id="clockDisplay" class="container mb-2"></div>
<div claas="container">
<div class="row">
<div class="napi col-md-6 mt-3">
<?php
if(!empty($_SESSION["cubic_cart"]))
{
$total = 0;
foreach($_SESSION["cubic_cart"] as $keys => $values)
{
?>
<?php
$total = $total + ($values["item_width"] * $values["item_length"]* $values["item_thickness"]);
}
?>
?>
<h3>Total: </h3>
<h2><?php echo number_format($total, 4); ?> m3</h2>
<?php
}
?>
</div>
</div>
</div>