Send specific attachment according to products sku / category

Hi,

for some of my product I need to send an additional pdf (not an invoice) to my customers.
With the help of this post:
https://wordpress.org/support/topic/attach-pdf-to-confirmation-email-for-specific-product/
I was able to attach an attachment to every order confirmation email.
Then I tried to change the code to filter by product sku.

In this post I found some infos about the variables that are available and read about some changes for WP3:
https://stackoverflow.com/questions/39401393/how-to-get-woocommerce-order-details/44708344#44708344

Here my code that unfortunately doesn’t work and is not attaching anything to the confirmation email anymore:

add_filter( 'woocommerce_email_attachments', 'webroom_attach_to_wc_emails', 10, 3);
function webroom_attach_to_wc_emails ( $attachments , $email_id, $order ) {

	// Avoiding errors and problems
    if ( ! is_a( $order, 'WC_Order' ) || ! isset( $email_id ) ) {
       return $attachments;
    }
  
	$file_path = get_stylesheet_directory() . '/Lizenzen/TestAttachment.pdf'; 
	$product_sku    = '1234';
	
	if( $email_id === 'customer_processing_order' ){
    		foreach ($order->get_items() as $item_key => $item ):
			$product        = $item->get_product();
			if ( $product_sku === $product->get_sku() ) {
				$attachments[] = $file_path;	
			}
		endforeach;
     	}
  return $attachments;
  }

Can anyone help me fixing it?
How would I have to change it to check for product category instead of SKU?

A general question would be: How can I debug this php code? Is there a way to show e.g. variables like email_id etc to check if the code gets correct values?

Thank you very much,
Toby