public function urlPlaceholder($iWidth = 100, $iHeight = 100, $border = 0)
    {
        if (!$this->isPermittedDimension($iWidth, $iHeight)) {
            throw new PermittedDimensionException(
                'CDN::urlPlaceholder() - Transformation of image to ' . $iWidth . 'x' . $iHeight . ' is not permitted'
            );
        }

        // --------------------------------------------------------------------------

        return $this->callDriver('urlPlaceholder', [$iWidth, $iHeight, $border]);
    }