Algolia搜索PHP API检索ObjectID

I'm using algolia search's php API to perform a name search. I would like for it to only return the objectIDs because I want to filter down the matches based on additional (external) criteria. I added the attributesToRetrieve parameter to the search() both as a string and as an array, but it still returns everything.

Any ideas? Thanks!

$index->search('value', array("attributesToRetrieve" => "objectID"));  

OR

 $index->search('value', array("attributesToRetrieve" => array("objectID")));