Cart is not updating nor incrementing an item on basket

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PHP

jQuery

HTML5

Hi Team

I have a shopping cart shows an item when its add or decrease, but when i refresh my page it only holds 1 item to the basket. There are no errors on the browser when inspecting. What could i be missing to my count?

What I have tried:

<?php
session_start();

var_dump($_SESSION['cart']);

require_once 'dbconn.php';

// add to cart
if(isset($_POST['id'])) {
  $product_id = $_POST['id'];
  if(isset($_SESSION['cart'][$product_id])) {
    $_SESSION['cart'][$product_id]++;
  } else {
    $_SESSION['cart'][$product_id] = 1;
  }
  echo count($_SESSION['cart']);
  exit;
}

// update cart
if(isset($_POST['update_cart'])) {
  $cart_data = $_POST['cart_data'];
  foreach($cart_data as $product_id => $quantity) {
    $_SESSION['cart'][$product_id] = $quantity;
  }
  echo 'success';
  exit;
}

?>

// jquery

$(document).ready(function() {
    update_cart_count();

    $('.add-to-cart-btn').click(function() {
        var product_id = $(this).data('id');

        $.ajax({
            url: 'add_to_cart.php',
            method: 'POST',
            data: {
                id: product_id
            },
            success: function(response) {
                update_cart_count();
            }
        });
    });

    function update_cart_count() {
        $.ajax({
            url: 'cart_count.php',
            method: 'GET',
            success: function(response) {
                $('#cart-count').html(response);
            }
        });
    }
});

// html code

<div id="basket-overview" class="navbar-collapse collapse d-none d-lg-block">
            <a href="basket.html" class="btn btn-primary navbar-btn">
            class="fa fa-shopping-cart">
           <span id="cart-count"><?php echo count($_SESSION['cart']); ?></span>
           </a>
          </div>

  <td><a href="#"><img src="img/detailsquare.jpg" alt="White Blouse Armani"></a></td>
  <td><a href="#">White Blouse Armani</a></td>
  <td>
    <div class="input-group">
      
        <button class="btn btn-default btn-minus" type="button">-</button>
      
      <input type="number" name="product1" value="2" min="1" class="form-control product-quantity" data-product-id="1">
      
        <button class="btn btn-default btn-plus" type="button">+</button>
      
    </div>
  </td>
  <td>R50</td>
  <td>%13</td>
  <td>R40</td>
  <td><a href="#">class="fa fa-trash-o"></a></td>
</tr>

// cart_count.php

<pre>session_start();
echo count($_SESSION['cart']);
exit;
?>