Add items to cart in php using sessions

$('.add-to-cart-mt').on('click', function(e){
        e.preventDefault();
        var id =$(this).attr('id');
        $('#add-to-cat-dialog').dialog({
            autoOpen:false,
            modal:true,
            hide:"pluff",
            show:"slide",
            height:200,
            open: function() { $(".ui-dialog-titlebar-close").hide(); },
            
            buttons:{
                "Add":function (){
                    $('#add-to-cat-dialog').dialog("close");
                    $.ajax({
                        url: 'add_to_cart.php',
                        data: {  productId : id },
                        
                        success: function (data) {
                            $('.top-cart-contain').empty();
                            $('.top-cart-contain').load("header_cart_summary.php");
                        },
                        
                        error :function (data, textStatus, jqXHR) {  }
                    });
                },
                
                "Cancel":function (){
                    $(this).dialog("close");
                }
            }
        });
        
        $('#add-to-cat-dialog').dialog("open");
    });

session_start();
require './database.php';

      if(!empty($_GET["productId"])) {
          $id=$_GET["productId"];
            $productById = $connection->query("SELECT * FROM products WHERE productId='$id';");
            if($productById && (mysqli_num_rows($productById)>0)){
                echo 'query ok';
                $productDetail= mysqli_fetch_assoc($productById);
                $itemArray = array(
                    $productDetail["productId"]=>array(
                        'name'=>$productDetail["productName"],
                        'id'=>$productDetail["productId"], 
                        'quantity'=>1,
                        'price'=>$productDetail["productPrice"],
                        'image'=>$productDetail["productsImage1"]
                        )
                    );
            
                if(!empty($_SESSION["cart_item"])) {
                    if(in_array($productDetail["productId"],array_keys($_SESSION["cart_item"]))) {
                        foreach($_SESSION["cart_item"] as $k => $v) {
                            if($productDetail["productId"] == $k) {
                                  if(empty($_SESSION["cart_item"][$k]["quantity"])) {
                                        $_SESSION["cart_item"][$k]["quantity"] = 0;
                                  }
                                  $_SESSION["cart_item"][$k]["quantity"] += 1;
                            }
                        }
                    } else {
                          $_SESSION["cart_item"] = array_merge($_SESSION["cart_item"],$itemArray);
                    }
                } else {
                      $_SESSION["cart_item"] = $itemArray;
                }
                
                //set product count in the bag
                if(!empty($_SESSION['cart_volume'])){
                    $_SESSION['cart_volume'] += 1;
                } else {
                    $_SESSION['cart_volume'] = 1;
                }
                
            }else {echo ''. mysqli_error($connection);}
            
      } else {
          echo 'no item';
}

[enter image description here][1]<li class="item col-lg-4 col-md-4 col-sm-6 col-xs-6 ">
                                <div class="product-item">
                                  <div class="item-inner">
                                    <div class="product-thumb has-hover-img">
                                      <figure> <a title="Ipsums Dolors Untra" href="single_product.html"> <img class="first-img" src="./productImages/<?php echo $item['productsImage1']; ?>" style="height: 250px;"  alt=""> <img class="hover-img" src="../images/products/img05.jpg" alt=""> </a></figure>
                                      <div class="pr-info-area animated animate2"><a href="quick_view.html" class="quick-view"><i class="fa fa-search"><span>Quick view</span></i></a> <a href="wishlist.html" class="wishlist"><i class="fa fa-heart"><span>Wishlist</span></i></a> <a href="compare.html" class="compare"><i class="fa fa-exchange"><span>Compare</span></i></a> </div>
                                    </div>
                                    <div class="item-info">
                                      <div class="info-inner">
                                        <div class="item-title"> <h4><a title="Ipsums Dolors Untra" href="single_product.html"><?php echo $item['productName']; ?></a></h4> </div>
                                        <div class="item-content">
                                          <div class="rating"> <i class="fa fa-star-o"></i> <i class="fa fa-star-o"></i> <i class="fa fa-star-o"></i> <i class="fa fa-star-o"></i> <i class="fa fa-star-o"></i> </div>
                                          <div class="item-price">
                                            <div class="price-box">
                                              <p class="special-price"> <span class="price-label">Special Price</span> <span class="price">Ksh. <?php echo number_format($item['productPrice'], 2) ; ?></span> </p>
                                              <!--<p class="old-price"> <span class="price-label">Regular Price:</span> <span class="price"> $567.00 </span> </p>-->
                                            </div>
                                          </div>
                                          <div class="pro-action">
                                              <button type="button" class="add-to-cart-mt" id="<?php echo $item['productId']; ?>"> <i class="fa fa-shopping-cart"></i><span> Add to Cart</span> </button>
                                          </div>
                                        </div>
                                      </div>
                                    </div>
                                  </div>
                                </div>
                              </li>