Example plugin for using multisite user meta

multisite-user-meta.php

<?php
/**
 * Plugin Name:     Multisite User Meta
 * Plugin URI:      https://foxland.fi/user-meta-in-wordpress-multisite/
 * Description:     Test multisite user meta with text value. 
 * Version:         1.0.0
 * Author:          Sami Keijonen
 * Author URI:      https://foxland.fi/
 * Text Domain:     multisite-user-meta
 * Domain Path:     /languages
 *
 * This program is free software; you can redistribute it and/or modify it under the terms of the GNU
 * General Public License version 2, as published by the Free Software Foundation. You may NOT assume
 * that you can use any other version of the GPL.
 *
 * This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without
 * even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 *
 * @package         Multisite User Meta
 * @author          Sami Keijonen <sami.keijonen@foxnet.fi>
 * @copyright       Copyright (c) Sami Keijonen
 * @license         http://www.gnu.org/licenses/old-licenses/gpl-2.0.html
 */

// Exit if accessed directly
if( !defined( 'ABSPATH' ) ) {
	exit;
}

/**
 * Adds setting in user profile.
 *
 * @since  1.0.0
 * @return void
 */
function multisite_user_meta_field( $user ) {
	
	// Get setting.
	$setting = multisite_user_meta_get_setting( $user->ID );
	
	?>

	<h3><?php esc_html_e( 'Multisite setting', 'multisite-user-meta' ); ?></h3>

	<table class="form-table">
		<tbody>
			<tr>
				<th scope="row"><label for="multisite_user_meta_setting"><?php esc_html_e( 'Test setting', 'multisite-user-meta' ); ?></label></th>
				<td>
					<input type="text" name="multisite_user_meta_setting" id="multisite_user_meta_setting" value="<?php if ( ! empty( $setting ) ) esc_attr_e( $setting ); ?>" class="multisite-user-meta medium-text multisite-user-meta-setting" />
					<p class="description"><?php esc_html_e( 'Set example setting.', 'multisite-user-meta' ); ?></p>
				</td>
			</tr>
		<tbody>
	</table>
	
<?php }
add_action( 'show_user_profile', 'multisite_user_meta_field' );
add_action( 'edit_user_profile', 'multisite_user_meta_field' );

/**
 * Save setting in user profile.
 *
 * @since  1.0.0
 * @return void
 */
function multisite_user_meta_save_field( $user_id ) {
	
	// Bail if we don't have permission to edit user.
	if ( ! current_user_can( 'edit_user', $user_id ) ) {
		return false;
	}
	
	// Get setting.
	$setting = isset( $_POST['multisite_user_meta_setting'] ) ? sanitize_text_field( $_POST['multisite_user_meta_setting'] ) : null;

	// Add user meta setting.
	update_user_option( $user_id, 'multisite_user_meta_setting', $setting );
	
}
add_action( 'personal_options_update', 'multisite_user_meta_save_field' );
add_action( 'edit_user_profile_update', 'multisite_user_meta_save_field' );

/**
 * Get user setting.
 *
 * @since  1.0.0
 * @return void
 */
function multisite_user_meta_get_setting( $user_id = 0 ) {
	
	// Get current user id if it's not set from function call
	if ( empty( $user_id ) && is_user_logged_in() ) {
		$user_id = get_current_user_id();
	}
		
	// Get setting
	$setting = get_user_option( 'multisite_user_meta_setting', $user_id );
		
	return $setting;

}