<button class="buy add-btn"  onclick="addCart(this);">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Добавить в корзину</button>

<input type="hidden" value="<?=$product->id?>" name="id"/>

<button class="buy add-btn"  onclick="addLiked(this);" style="background-image: none;">Добавить в избранное</button>

function addCart(btn) {
    $this= $(btn);
    $(".shopCart").css({
                backgroundImage:"url(/images/icon/ico-Cartfull.png)",
                backgroundPosition:"14px center",
                backgroundRepeat:"no-repeat"
            });
    var cart_offset = $("#cart").offset(); //координаты корзины
    var cart_img = $this.parents('.item').find('img');
    $('<style id="transferEffect" type="text/css">' + // Add new one
        '.ui-effects-transfer { background-image: url(' + $(cart_img).attr('src') + '); }' +
        '</style>').appendTo('head');
    $(cart_img).effect("transfer",{ clone: cart_img, to: $("#cart") }, 750);

    var id = $this.siblings('input[name=id]').val();

    $.ajax({
        url: '/cart/add_cart',
        dataType : "json",
        type : 'post',
        data : {
            'id':id
        },
        success: function (data, textStatus) {
            $(".total_count").text(data.total_count+" шт.");
            $(".sum").text(data.total_price);
            $('.name_modal').html(data.addProduct.title);
            $('.price_modal').html(data.addProduct.price);
            $('.img_modal').attr('src','/'+data.addProduct.img);
            $('#myModal').reveal({});
        }
    });
}



function addLiked(btn) {
    $this= $(btn);



    var id = $this.siblings('input[name=id]').val();
    alert(id);

}

Uncaught ReferenceError: addLiked is not defined