Customize email subject in WooCommerce with vendor store-name (Dokan) - php

Question

We found this question https://wordpress.stackexchange.com/questions/320924/woocommerce-order-processing-email-subject-not-changing and it's working fine. We now want to extend this by display the vendor on a order in the order completed mail.
But we are not able to output the vendor store name.
Is there a obvious error in our code?
add_filter( 'woocommerce_email_subject_customer_completed_order',
'change_completed_email_subject', 1, 2 );
function change_completed_email_subject( $subject, $order ) {
global $woocommerce;
// Order ID
$order->get_items();
// Author id
$author_id = $product->post->post_author;
// Shopname
$vendor = dokan()->vendor->get( $author_id );
$shop_name = $vendor->get_shop_name();
// Blogname
$blogname = wp_specialchars_decode(get_option('blogname'), ENT_QUOTES);
// Output subject
$subject = sprintf( '%s, Deine %s Bestellung (#%s) wurde versendet! Vendor: %s', $order->billing_first_name, $blogname, $order->get_order_number(), $shop_name );
return $subject;
}
Update:
I already tried to get the name via $shop_name = dokan()->vendor->get( $author_id )->get_shop_name(); but no success.

Answer 1

The use of global $woocommerce is not necessary
You use $order->get_items();, but don't do anything with it
$product is not defined
Use $order->get_billing_first_name() VS $order->billing_first_name
So you get:
function filter_woocommerce_email_subject_customer_completed_order( $subject, $order ) {
// Empty array
$shop_names = array();
// Loop through order items
foreach ( $order->get_items() as $item ) {
// Get product object
$product = $item->get_product();
// Author id
$author_id = $product->post->post_author;
// Shopname
$vendor = dokan()->vendor->get( $author_id );
$shop_name = $vendor->get_shop_name();
// OR JUST USE THIS FOR SHOPNAME
// Shop name
// $shop_name = dokan()->vendor->get( $author_id )->get_shop_name();
// NOT in array
if ( ! in_array( $shop_name, $shop_names ) ) {
// Push to array
$shop_names[] = $shop_name;
}
}
// Blogname
$blogname = wp_specialchars_decode(get_option('blogname'), ENT_QUOTES);
// Set subject
$subject = sprintf( __( '%s, Deine %s Bestellung (#%s) wurde versendet! Vendor: %s', 'woocommerce' ), $order->get_billing_first_name(), $blogname, $order->get_order_number(), implode( ', ', $shop_names ) );
// Return
return $subject;
}
add_filter( 'woocommerce_email_subject_customer_completed_order', 'filter_woocommerce_email_subject_customer_completed_order', 10, 2 );