Restrict total cart quantity to specific multiples in WooCommerce

Question

I want the number of products in the customer cart to be a multiple of 6 or 12 or 4. I found this code for multiplier 6 in the forum, but it needs to be edited for other coefficients.

Based on Set item quantity to multiples of “x” for products in a specific category in Woocommerce code thread:

// check that cart items quantities totals are in multiples of 6
add_action( 'woocommerce_check_cart_items', 'woocommerce_check_cart_quantities' );
function woocommerce_check_cart_quantities() {
   $multiples = 6;
   $total_products = 0;
   foreach ( WC()->cart->get_cart() as $cart_item_key => $values ) {
       $total_products += $values['quantity'];
   }
   if ( ( $total_products % $multiples ) > 0 )
       wc_add_notice( sprintf( __('You need to buy in quantities of %s products', 'woocommerce'), $multiples ), 'error' );
}

Based on Woocommerce Order Quantity in Multiples answer code:

// Limit cart items with a certain shipping class to be purchased in multiple only
add_action( 'woocommerce_check_cart_items', 'woocommerce_check_cart_quantities_for_class' );
function woocommerce_check_cart_quantities_for_class() {
    $multiples = 6;
    $class = 'bottle';
    $total_products = 0;
    foreach ( WC()->cart->get_cart() as $cart_item_key => $values ) {
        $product = get_product( $values['product_id'] );
        if ( $product->get_shipping_class() == $class ) {
            $total_products += $values['quantity'];
        }
    }
    if ( ( $total_products % $multiples ) > 0 )
        wc_add_notice( sprintf( __('You need to purchase bottles in quantities of %s', 'woocommerce'), $multiples ), 'error' );
} 

How to handle multiples of 6 or 12 or 4 instead of just one?

Answer 1

As 12 is already a multiple of 4 or 6, you need only to check for items multiples of 4 or 6 using the following code (commented):

add_action( 'woocommerce_check_cart_items', 'check_cart_items_quantities_conditionally' );
function check_cart_items_quantities_conditionally() {
    $multiples   = array(4, 6); // Your defined multiples in an array
    $items_count = WC()->cart->get_cart_contents_count(); // Get total items cumulated quantity
    $is_multiple = false; // initializing

    // Loop through defined multiples
    foreach ( $multiples as $multiple ) {
       if ( ( $items_count % $multiple ) == 0 ) {
           $is_multiple = true; // When a multiple is found, flag it as true
           break; // Stop the loop
        }
    }
    
    // If total items cumulated quantity doesn't match with any multiple
    if ( ! $is_multiple ) {
        // Display a notice avoiding checkout
        wc_add_notice( sprintf( __('You need to buy in quantities of %s products', 'woocommerce'), implode(' or ', $multiples) ), 'error' );
    }
}