Fetching articles from the NY Times API

Analogous to yesterday's code snippet for the Guardian Open Platform, here's how to fetch articles (extracts, not the full text) from the New York Times Article Search API: 
<?php
define('API_KEY', 'YOUR_API_KEY');
define('API_URL', 'http://api.nytimes.com/svc/search/v1');
$n = 10;
$i = 0;
do{
  $start = $i * $n;
  print "$start\n";
  
  $params = array(
    'api-key' => API_KEY,
    'query' => 'des_facet:[SCIENCE AND TECHNOLOGY]',
    'offset' => $i,
    'fields' => 'byline,body,date,title,url,des_facet',
    );
  
  $data = json_decode(file_get_contents(API_URL . '/article?' . http_build_query($params)));
  foreach ($data->results as $item)
    file_put_contents(sprintf('articles/%s.js', md5($item->url)), json_encode($item)); 
  
  sleep(1);
} while ($start total && ++$i

Sadly it doesn't seem possible to get the categories assigned to an individual article - adding "facets=des_facet" only provides the sum of the categories across the 10 results. (see comment below).

The same Nature News story as before has these similar articles:

  1. OBSERVATORY; Galaxies Made Simple, or at Least Less Complicated
  2. THE YEAR IN IDEAS; Dark Energy
  3. Q&A
  4. Galaxies Twice as Bright as They Seem, Study Finds
  5. Stars Suggest a Quark Twist And a New Kind of Matter
  6. OBSERVATORY; Galactic Puzzle Solved? Threads Tie It Together
  7. VAST 'HOLE' IN SPACE APPEARS TO DEFY THEORIES
  8. ESSAY; Computing, 2016: What Won't Be Possible?
  9. THE NOVEMBER REPORT; Scientists In Waiting
  10. A Puzzle Finally Makes the 'Cosmic Figures' Fit

and the 50 most similar articles have these categories:

  1. SPACE [17]
  2. PHYSICS [14]
  3. RESEARCH [6]
  4. STARS AND GALAXIES [6]
  5. UNIVERSE [6]
  6. REVIEWS [5]
  7. ASTRONOMY AND ASTROPHYSICS [5]
  8. BOOKS AND LITERATURE [4]
  9. HUBBLE SPACE TELESCOPE [4]
  10. COMPUTERS AND THE INTERNET [4]
  11. EDUCATION AND SCHOOLS [3]
  12. SOCIAL CONDITIONS AND TRENDS [3]
  13. TELESCOPES AND OBSERVATORIES [3]
  14. UNITED STATES ECONOMY [2]
  15. BIOGRAPHICAL INFORMATION [2]
  16. GENETICS AND HEREDITY [2]
  17. THIRD WORLD AND DEVELOPING COUNTRIES [2]
  18. PHOTOGRAPHY [2]
  19. ACCIDENTS AND SAFETY [2]