Extend User Profiles and Restrict Access to a Single Admin Only

admin_user_caps.php

<?php

if (!class_exists('Admin_User_Caps')) {

  class Admin_User_Caps 
  {
    function __construct()
    {
      add_action('admin_init', array($this, 'extend_user_profile'));
    }  

    public function extend_user_profile() 
    {
      // Add field for user access to the Plugin.
      add_action('user_new_form', array($this, 'register_profile_fields')); // /wp-admin/user-new.php
      add_action('edit_user_profile', array($this, 'register_profile_fields')); // /wp-admin/user-edit.php?user_id=1
      // add_action('show_user_profile', array($this, 'register_profile_fields')); // /wp-admin/profile.php

      // Save user access to the Plugin as usermeta.
      add_action('user_register', array($this, 'save_profile_fields')); // /wp-admin/user-new.php
      add_action('edit_user_profile_update', array($this, 'save_profile_fields')); // /wp-admin/user-edit.php?user_id=1
      // add_action('personal_options_update', 'cp_save_profile_fields'); // /wp-admin/profile.php
    }

    public function register_profile_fields($user) 
    {
      /** 
       * We want to restrict Administrators to change the BKPC access themselves.
       * 
       * If Admin use is created without access to the Plugin then they won't
       * be able to Add, Edit or Update others with this options.
       */ 
      $current_user_id  = get_current_user_id();
      $current_user_can = get_user_meta($current_user_id, 'user_can_access_plugin_name')[0];
      $update_user_can  = get_user_meta($user->ID, 'user_can_access_plugin_name')[0];

      if (
        !current_user_can('administrator', $current_user_id) 
        || $current_user_can === null) {
        return false;
      }
      ?>
        <h2><br />Extend User Options</h2>
        <table class="form-table">
          <tr>
            <th>
              <label for="user_can_access_plugin_name">User can have access<br /> to Plugin?</label>
            </th>
            <td>
              <?php if ($update_user_can !== null) : ?>
                <input type="checkbox" class="regular-text" name="user_can_access_plugin_name" value="1" id="user_can_access_plugin_name" checked />
              <?php else : ?>
                <input type="checkbox" class="regular-text" name="user_can_access_plugin_name" value="1" id="user_can_access_plugin_name" />
              <?php endif; ?>
              <em>This is useful when you want to restrict access to Plugin for other Administrators.</em>
            </td>
          </tr>
        </table>
      <?php 
    }

    public function save_profile_fields($update_user_id) 
    {
      $current_user_id  = get_current_user_id();
      $current_user_can = get_user_meta($current_user_id, 'user_can_access_plugin_name')[0];

      if (
        !current_user_can('administrator', $current_user_id) 
        || $current_user_can === null) {
        return false;
      }

      update_user_meta($update_user_id, 'user_can_access_plugin_name', $_POST['user_can_access_plugin_name']);
    }
  }

  new Admin_User_Caps;
}