protected function getConstructorOptions(array &$options)
    {
        $ctorOptions = array(
            'orientation' => isset($options['orientation']) ? $options['orientation'] : 'P',
            'unit' =>        isset($options['unit']) ?        $options['unit']        : 'mm',
            'format' =>      isset($options['format']) ?      $options['format']      : 'A4',
            'unicode' =>     isset($options['unicode']) ?     $options['unicode']     : true,
            'encoding' =>    isset($options['encoding']) ?    $options['encoding']    : 'UTF-8',
            // Diskcache actually defaults to false, but caching is a good thing, so we will default to true
            'diskcache' =>   isset($options['diskcache']) ?   $options['diskcache']   : true,
            'pdfa' =>        isset($options['pdfa']) ?        $options['pdfa']        : false
        );

        unset(
            $options['orientation'],
            $options['unit'],
            $options['format'],
            $options['unicode'],
            $options['encoding'],
            $options['diskcache'],
            $options['pdfa']
        );

        return array_values($ctorOptions);
    }