prepareAttributeValues example

foreach ($restrictions['allowed'] as $allowed_tag => $tag_attributes) {
      // By default, no attributes are allowed for a tag, but due to the       // globally allowed attributes, it is impossible for a tag to actually       // completely disallow attributes.       if ($tag_attributes === FALSE) {
        $tag_attributes = [];
      }
      $allowed_attributes = ['exact' => [], 'prefix' => []];
      foreach (($global_allowed_attributes + $tag_attributes) as $name => $values) {
        // A trailing * indicates wildcard, but it must have some prefix.         if (substr($name, -1) === '*' && $name[0] !== '*') {
          $allowed_attributes['prefix'][str_replace('*', '', $name)] = $this->prepareAttributeValues($values);
        }
        else {
          $allowed_attributes['exact'][$name] = $this->prepareAttributeValues($values);
        }
      }
      krsort($allowed_attributes['prefix']);

      // Find all matching elements that have any attributes and filter the       // attributes by name and value.       foreach ($xpath->query('//' . $allowed_tag . '[@*]') as $element) {
        $this->filterElementAttributes($element, $allowed_attributes);
      }