/**
  * Add individual plugin to our collection of plugins.
  *
  * If the required keys are not set or the plugin has already
  * been registered, the plugin is not added.
  *
  * @since 2.0.0
  *
  * @param array|null $plugin Array of plugin arguments or null if invalid argument.
  * @return null Return early if incorrect argument.
  */
 public function register($plugin)
 {
     if (empty($plugin['slug']) || empty($plugin['name'])) {
         return;
     }
     if (empty($plugin['slug']) || !is_string($plugin['slug']) || isset($this->plugins[$plugin['slug']])) {
         return;
     }
     $defaults = array('name' => '', 'slug' => '', 'source' => 'repo', 'required' => false, 'version' => '', 'force_activation' => false, 'force_deactivation' => false, 'external_url' => '', 'is_callable' => '');
     // Prepare the received data.
     $plugin = wp_parse_args($plugin, $defaults);
     // Standardize the received slug.
     $plugin['slug'] = $this->sanitize_key($plugin['slug']);
     // Forgive users for using string versions of booleans or floats for version number.
     $plugin['version'] = (string) $plugin['version'];
     $plugin['source'] = empty($plugin['source']) ? 'repo' : $plugin['source'];
     $plugin['required'] = TGMPA_Utils::validate_bool($plugin['required']);
     $plugin['force_activation'] = TGMPA_Utils::validate_bool($plugin['force_activation']);
     $plugin['force_deactivation'] = TGMPA_Utils::validate_bool($plugin['force_deactivation']);
     // Enrich the received data.
     $plugin['file_path'] = $this->_get_plugin_basename_from_slug($plugin['slug']);
     $plugin['source_type'] = $this->get_plugin_source_type($plugin['source']);
     // Set the class properties.
     $this->plugins[$plugin['slug']] = $plugin;
     $this->sort_order[$plugin['slug']] = $plugin['name'];
     // Should we add the force activation hook ?
     if (true === $plugin['force_activation']) {
         $this->has_forced_activation = true;
     }
     // Should we add the force deactivation hook ?
     if (true === $plugin['force_deactivation']) {
         $this->has_forced_deactivation = true;
     }
 }

 /**
  * Helper function: Validate a value as boolean
  *
  * @since 2.5.0
  *
  * @static
  *
  * @param mixed $value Arbitrary value.
  * @return bool
  */
 public static function validate_bool($value)
 {
     if (!isset(self::$has_filters)) {
         self::$has_filters = extension_loaded('filter');
     }
     if (self::$has_filters) {
         return filter_var($value, FILTER_VALIDATE_BOOLEAN);
     } else {
         return self::emulate_filter_bool($value);
     }
 }