public function convertToMM($css, $old=0.)
    {
        $css = trim($css);
        if (preg_match('/^[0-9\.\-]+$/isU', $css))        $css.= 'px';
        if (preg_match('/^[0-9\.\-]+px$/isU', $css))      $css = 25.4/96. * str_replace('px', '', $css);
        else if (preg_match('/^[0-9\.\-]+pt$/isU', $css)) $css = 25.4/72. * str_replace('pt', '', $css);
        else if (preg_match('/^[0-9\.\-]+in$/isU', $css)) $css = 25.4 * str_replace('in', '', $css);
        else if (preg_match('/^[0-9\.\-]+mm$/isU', $css)) $css = 1.*str_replace('mm', '', $css);
        else if (preg_match('/^[0-9\.\-]+%$/isU', $css))  $css = 1.*$old*str_replace('%', '', $css)/100.;
        else                                              $css = null;

        return $css;
    }