plugin development – Print Current User Display Name

I am trying to print the current user display name with the following. Any help how to make it work?

function filter_product_pwb_brand($value, $post_id, $form_data){
//$value is the post field value to return, by default it is empty. If you are filtering a taxonomy you can return either slug/id/array. in case of ids make sure to cast them integers.(see for more information.)
//$post_id is the ID of the post to which the form values are being mapped to
// $form_data is the submitted form data as an array of field-name=>value pairs
return $value;