Wordpress: Set display name with 'first name'

gistfile1

/* Sets the user's display name (always) to first name last name, when it's available **************************************
** http://stackoverflow.com/questions/9326315/wordpress-change-default-display-name-publicy-as-for-all-existing-users *** */
/* !아이디 대신 이름으로 나타내기 ********************************************************************************************* */
/* Sets the user's display name (always) to first name last name, when it's avail. */
add_action ('admin_head','make_display_name_f_name_last_name');
function make_display_name_f_name_last_name(){
    $users = get_users(array('fields'=>'all'));
    foreach($users as $user){   
        $user = get_userdata($user->ID);
        $display_name = $user->first_name;
        if($display_name!=' ') wp_update_user( array ('ID' => $user->ID, 'display_name' => $display_name) );
            else wp_update_user( array ('ID' => $user->ID, 'display_name' => $user->display_login) );
        if($user->display_name == '') 
            wp_update_user( array ('ID' => $user->ID, 'display_name' => $user->display_login) );
    }   
}