function sort_by_last_name($a, $b) {
  // remove titles and split names, sorting on last name
  $a['name'] = trim($a['name']);
  $b['name'] = trim($b['name']);
  $last_name1 = end(explode(' ', strpos($a['name'], ',') ? substr($a['name'], 0, strpos($a['name'], ',')) : $a['name']));
  $last_name2 = end(explode(' ', strpos($b['name'], ',') ? substr($b['name'], 0, strpos($b['name'], ',')) : $b['name']));

  if(substr_count($a['name'], ' ') > 1) {
    $last_name1 = end(explode(' ', strpos($a['name'], ' I') ? substr($a['name'], 0, strrpos($a['name'], ' I')) : $a['name']));
  }
  if(substr_count($b['name'], ' ') > 1) {
    $last_name2 = end(explode(' ', strpos($b['name'], ' I') ? substr($b['name'], 0, strrpos($b['name'], ' I')) : $b['name']));
  }
  return strcasecmp($last_name1, $last_name2);
}