Code-Snippets-Functions/Execute a function on a child site/WooCommerce/allow-only-one-product-in-cart.txt

// Used to calculate totals
function action_woocommerce_before_calculate_totals( $cart ) {
    if ( is_admin() && ! defined( 'DOING_AJAX' ) )
        return;

    if ( did_action( 'woocommerce_before_calculate_totals' ) >= 2 )
        return;
    
    // Get cart
    $get_cart = $cart->get_cart();

    // Solution for PHP 7.3 and up
    foreach ( $get_cart as $cart_item_key => $cart_item ) {
        // NOT last element
        if ( $cart_item_key !== array_key_last( $get_cart ) ) {
            // Remove a cart item
            $cart->remove_cart_item( $cart_item_key );
        }
    }
}
add_action( 'woocommerce_before_calculate_totals', 'action_woocommerce_before_calculate_totals', 10, 1 );