Get in WooCommerce cart the product ID of a cart item

To get the product ID of each cart item in the foreach loop (for a simple product):

foreach( WC()->cart->get_cart() as $cart_item ){
    $product_id = $cart_item['product_id'];
}

If it's a variable product, to get the variation ID:

foreach( WC()->cart->get_cart() as $cart_item ){
    $variation_id = $cart_item['variation_id'];
}

Or for both cases (where $cart_item['data'] is the WC_Product Object in Woocommerce 3+):

foreach( WC()->cart->get_cart() as $cart_item ){
    // compatibility with WC +3
    if( version_compare( WC_VERSION, '3.0', '<' ) ){
        $product_id = $cart_item['data']->id; // Before version 3.0
    } else {
        $product_id = $cart_item['data']->get_id(); // For version 3 or more
    }
}

Update: Using Product ID outside the loop

1) Breaking the loop (Just to get the first item ID (product ID) of cart):

foreach( WC()->cart->get_cart() as $cart_item ){
    $product_id = $cart_item['product_id'];
    break;
}

You can use directly $product_id variable of the first item in cart.

2) Using an array of product IDs (one for each item in cart).

$products_ids_array = array();

foreach( WC()->cart->get_cart() as $cart_item ){
    $products_ids_array[] = $cart_item['product_id'];
}

• To get the 1st item product ID: $products_ids_array[0];
• To get the 2nd item product ID: $products_ids_array[1]; etc…

To check product categories or product tags in cart item use WordPress has_term() like:

foreach( WC()->cart->get_cart() as $cart_item ){
    // For product categories (term IDs, term slugs or term names)
    if( has_term( array('clothing','music'), 'product_cat', $cart_item['product_id'] ) ) {
        // DO SOMETHING
    }

    // For product Tags (term IDs, term slugs or term names)
    if( has_term( array('clothing','music'), 'product_tag', $cart_item['product_id'] ) ) {
        // DO SOMETHING ELSE
    }
}

We always use $cart_item['product_id'] as we get the parent variable product when a cart item is a product variation.

Product variations don't handle any custom taxonomy as product categories and product tags

To get the product id from the WC_Subscription Object, you will need to loop through order items (as you can have many) using the method get_items() like:

$order_items = $subscription->get_items();

// Loop through order items
foreach ( $order_items as $item_id => $item ) {
    // Get the WC_Product_Subscription Object
    $product = $item->get_product();

    // To get the subscription variable product ID and simple subscription  product ID
    $product_id = $item->get_product_id();

    // To get the variation subscription product ID
    $variation_id = $item->get_variation_id();

    // Or to get the simple subscription or the variation subscription product ID
    $_product_id = $product->get_id();
}

Tested and works.

Related:

• How to get the user ID from a WC_Subscription instance Object
• How to get Order Details from a WC_Subscription instance Object

You should use woocommerce_after_cart_item_quantity_update action hook that has 4 arguments. But when quantity is changed to zero, woocommerce_before_cart_item_quantity_zero action hook need to be used instead (and has 2 arguments).

Below is a working example that will limit the updated quantity to a certain amount and will display a custom notice:

add_action( 'woocommerce_after_cart_item_quantity_update', 'limit_cart_item_quantity', 20, 4 );
function limit_cart_item_quantity( $cart_item_key, $quantity, $old_quantity, $cart ){
    if( ! is_cart() ) return; // Only on cart page

    // Here the quantity limit
    $limit = 5;

    if( $quantity > $limit ){
        // Change the quantity to the limit allowed
        $cart->cart_contents[ $cart_item_key ]['quantity'] = $limit;
        // Add a custom notice
        wc_add_notice( __('Quantity limit reached for this item'), 'notice' );
    }
}

This code goes on function.php file of your active child theme (or theme). Tested and works.

As this hook is located in WC_Cart set_quantity() method, is not possible to use that method inside the hook, as it will throw an error.

To trigger some action when quantity is set to Zero use:

add_action( 'woocommerce_before_cart_item_quantity_zero', 'action_before_cart_item_quantity_zero', 20, 4 );
function action_before_cart_item_quantity_zero( $cart_item_key, $cart ){
    // Your code goes here
}