WooCommerce: Add notices to the cart/checkout page when specific categories are together in the cart.

functions.php

// Hook in to add notices to the cart and checkout pages
add_action( 'woocommerce_before_cart_contents', 'wc_ninja_add_cart_notice' );
add_action( 'woocommerce_before_checkout_form', 'wc_ninja_add_cart_notice' );

/**
 * Add the cart/checkout notice
 */
function wc_ninja_add_cart_notice() {
	$message = "Sorry, you cannot purchase products from the clothing and music category together.";

	// Exchange "clothing" and "music" for your own categories.
	if ( wc_ninja_categories_in_cart( array( 'clothing', 'music' ) ) ) {
		echo '<div id="woocommerce-cart-notice-gateways-unavailable" class="woocommerce-cart-notice woocommerce-error">' . $message.  '</div>';
	}

}

/**
 * Check if specific product categories are in the cart.
 *
 * @param array $categories
 */
function wc_ninja_categories_in_cart( $categories ) {
	// Products currently in the cart
	$cart_ids = array();

	// Categories currently in the cart
	$cart_categories = array();

	// Find each product in the cart and add it to the $cart_ids array
	foreach( WC()->cart->get_cart() as $cart_item_key => $values ) {
		$cart_product = $values['data'];
		$cart_ids[]   = $cart_product->id;
	}

	// Connect the products in the cart w/ their categories
	foreach( $cart_ids as $id ) {
		$products_categories = get_the_terms( $id, 'product_cat' );

		// Loop through each product category and add it to our $cart_categories array
		foreach ( $products_categories as $products_category ) {
			$cart_categories[] = $products_category->slug;
		}
	}

	// If all of the special categories are in the cart, return true.
	if ( ! array_diff( $categories, $cart_categories ) ) {
		return true;
	} else {
		return false;
	}
}