sparql-collection-outline.php

<?php
function getCollectionData($collection_pid) {
  $user = user_load(1);
  $repository = islandora_get_tuque_connection($user);
  $query = <<<EOQ
SELECT ?pid ?label ?cmodel
FROM <#ri>
WHERE {
?pid  <info:fedora/fedora-system:def/relations-external#isMemberOfCollection> <info:fedora/$collection_pid> ;
      <fedora-model:label> ?label ;
      <fedora-model:hasModel> ?cmodel
      FILTER (?cmodel != <info:fedora/fedora-system:FedoraObject-3.0>)
}
EOQ;
  $results = $repository->repository->ri->sparqlQuery($query);
  $subCollections = array();
  $recordCount = 0;
  foreach($results as $result) {
    if ($result['cmodel']['value'] != 'islandora:collectionCModel') {
      $recordCount = $recordCount + 1;
    }
    else {
      $subCollections[] = $result['pid']['value'];
    }
  }
  $ret = array('records' => $recordCount, 'collections' => $subCollections);
  $collection_title = islandora_object_load($collection_pid)->label;
  $msg = "<a href=\"/islandora/object/$collection_pid\">$collection_title [$recordCount]</a>\n<ul>\n";
  foreach ($subCollections as $subCollection) {
    $submsg = getCollectionData($subCollection);
    $msg .= "<li>$submsg</li>\n";
  }
  $msg .= "</ul>";
  return $msg;
}

// Initiate function with target collection PID of your choice
echo getCollectionData('islandora:root');
?>

This would work well inside a custom module (providing content for a block, for example).
Is the 'isMemberOfCollection' predicate transitive? If so you might be able to get all the subcollections using just one SPARQL query.