Make WP First name & last name as default display name

/** With WPUF it's better to set the profile update functions priority high **/
/** Alternative solution as this plugin - https://wordpress.org/plugins/force-first-last/ **/

/* First name & last name as default display name. */
add_action( 'profile_update', 'set_display_name', 20 );

function set_display_name( $user_id ) {

    $data = get_userdata( $user_id );

    if($data->first_name) {

        remove_action( 'profile_update', 'set_display_name', 10 ); // profile_update is called by wp_update_user, so we need to remove it before call, to avoid infinite recursion
        wp_update_user(
            array (
                'ID' => $user_id,
                'display_name' => "$data->first_name $data->last_name"
            )
        );
        add_action( 'profile_update', 'set_display_name', 20 ); // set the priority high to override other plugins
    }
}