woocommerce_email_attachments hook with empty $order argument in Woocommerce

Your developer has been using maybe this woocommerce official snippet (That is outdated as it is 3 years old) and it uses woocommerce_email_attachments filter hook.

This filter hook is defined:

• one time (1) in WC_Email class (with the $order argument)
• three times (3) in WC_emails Class (without the $order argument)

So the WC_Order Object $order is only defined in WC_Email class as third argument.

That means that this outdated official code need to be tweaked as follow:

add_filter( 'woocommerce_email_attachments', 'woocommerce_emails_attach_downloadables', 10, 3 );function woocommerce_emails_attach_downloadables( $attachments, $email_id, $order ) {    // Avoiding errors and problems    if ( ! is_a( $order, 'WC_Order' ) || ! isset( $email_id ) ) {        return $attachments;    }    // ===>  Your custom code goes Here  <===    return $attachments;}

This should work avoiding problems.

So as you are asking, this hook is also used on the following specific product notifications with really different hook arguments:

• On "Low stock" notification (with 'low_stock' and $product as 2nd & 3rd arguments)
• On "No stock" notification (with 'no_stock' and $product as 2nd & 3rd arguments)
• On "Backorder" notification (with 'backorder' and $args as 2nd & 3rd arguments)

So here the 2nd argument is the stock status $stock_status and the 3rd argument is the WC_Product object $product (or an array $args).

Then In those 3 notifications, the $order object doesn't exist as it can be a WC_Product Object, an array, an empty array, or null.