这应该让你开始。我已经使用了5分钟的超时时间来允许网站空闲。您可以使用脚本提高准确性（如果当前用户是管理员）（&；每隔几分钟ping一次AJAX请求以更新admin_last_seen 时间戳。

/**
 * Check if the admin was last online at least 5 minutes ago.
 * 
 * @return bool
 */
function wpse_140253_is_admin_online() {
    if ( false === $last_seen = get_option( \'admin_last_seen\' ) )
        update_option( \'admin_last_seen\', $last_seen = 0 );
    elseif ( $last_seen )
        return $last_seen + 5 * MINUTE_IN_SECONDS > time();
    return false;
}

/**
 * Update "admin_last_seen" timestamp.
 * 
 * @link http://wordpress.stackexchange.com/q/140253/1685
 */
function wpse_140253_update_admin_online_status() {
    if ( current_user_can( \'manage_options\' ) /* Might be better to check user ID if you have multiple admins */ ) {
        if ( ! $last_seen = get_option( \'admin_last_seen\' ) )
            $last_seen = 0;

        // Only make a database update if last seen greater than timeout.
        if ( $last_seen + 5 * MINUTE_IN_SECONDS < time() )
            update_option( \'admin_last_seen\', time() ); 
    }       
}

add_action( \'init\', \'wpse_140253_update_admin_online_status\' );

<?php if ( wpse_140253_is_admin_online() ) : ?>
     <div class="message">Admin is online!</div>
<?php endif ?>