$(document).ready(function(){
        $(".addCart").click(function(){
            $.post(".addCart.val()",function(data){
                $("#test").html(data).show();
            });
        });
    });

<div id="test" style="background-color: grey"></div>

   <?php while ($row = mysqli_fetch_array($products)):?>
            <div class="span3">
                <div class="product">
                    <div class="product-img">
                        <div class="picture">
                            <img src="images/<?php echo $row['image'] ?>" alt="" width="540" height="374" />
                            <div class="img-overlay">
                                <a href="#" class="btn more btn-primary">توضیحات بیشتر</a>
                                <button value="<?php echo $row['id']?>" class="btn buy btn-danger addCart">اضافه به سبد خرید</button>
                            </div>
                        </div>
                    </div>
                    <div class="main-titles no-margin">
                        <h4 class="title"><?php echo $row['price'] ?></h4>
                        <!--<h5 class="no-margin">محصول ویژه 434</h5>-->
                    </div>
                    <p class="desc"><?php echo $row['name']?></p>
                    <p class="center-align stars">
                        <span class="icon-star stars-clr"></span>
                        <span class="icon-star stars-clr"></span>
                        <span class="icon-star"></span>
                        <span class="icon-star"></span>
                        <span class="icon-star"></span>

                    </p>
                </div>
            </div> <!-- /product -->
 <?php endwhile;?>

$(".addCart").click(function(event){
        var id = event.target.val();
    });