由于Woocommerce 3，您的实际代码有点过时。在以下内容中，您将在电子邮件主题中获得所有产品名称（coma分隔）。

add_filter(\'woocommerce_email_subject_new_order\', \'change_admin_email_subject\', 10, 2);
function change_admin_email_subject( $subject, $order ) {
    $products_names = array();

    foreach ( $order->get_items() as $item ) {
        $products_names[] = $item->get_name();
    }

    return sprintf( \'[%s] New Customer Order (#%s) of %s from %s %s\', 
        wp_specialchars_decode(get_option(\'blogname\'), ENT_QUOTES), 
        $order->get_id(), 
        implode(\', \', $products_names),
        $order->get_billing_first_name(),  
        $order->get_billing_last_name()
    );
}