$xQuery = $xml->comiclist->comic;

function cmp ($a, $b) {
     return strcmp(
         $a->mainsection->series->sortname,
         $b->mainsection->series->sortname
     );
}

usort($xQuery, "cmp");

foreach ($xQuery as $comic) :

<sortname>New Avengers: Illuminati, Vol. 2</sortname>
<sortname>New Avengers: Illuminati, Vol. 2</sortname>
<sortname>Nova</sortname>
<sortname>New X-Men</sortname>
<sortname>Nation X</sortname>
<sortname>Namor, The Sub-Mariner Annual</sortname>

$array = iterator_to_array($xQuery, false);

usort($array, "cmp");

foreach ($array as $comic) ...