You get an error because you are mixing up your quotes. If you start a string with double quotes you need to close the string with double quotes as well.
$xmlDoc->load("ds_".$lang1."_".$subject.".xml");
or
$xmlDoc->load('ds_'.$lang1.'_'.$subject.'.xml');
or
$xmlDoc->load("ds_{$lang1}_{$subject}.xml");