If you have a number of JSON documents with nested elements such as 'responseId' below and you want to know how many documents have a responseId:
{"result":{"responseId":"xyz"}}
{"result":NULL}
{"result":{"responseId":"abc"}}
I was not having luck with the following format
<?php
// trying to get the count of documents where responseId is not equal to NULL (did not work for me)
$intCount = $collection->count(['result' => ['responseId' => ['$ne' => NULL]]]);
?>
Instead I needed to use a period between the JSON elements:
<?php
// get the count of documents where responseId is not equal to NULL
$intCount = $collection->count(['result.responseId' => ['$ne' => NULL]]);
?>