<button onclick="cart.addcart('<?php echo $product['product_id']; ?>');" class="btn btn-shopping-cart" value="<?php echo $button_cart; ?>">
<button onclick="cart.addcart('<?php echo $product['product_id']; ?>'); this.value = 'В корзине';" class="btn btn-shopping-cart" value="<?php echo $button_cart; ?>">