PHP Mongo Query查找空数组

I'm trying to find the empty-array subdocument records from a collection,

In Mongo Shell, below query returns 3 records,

db.test.find({"campaigns.postcode":{"$gt":{}}}).count();

same in PHP returns all the records from test collection,

$mongo->$dbname->$colname->count(array('campaigns.postcode'=>array('$gt'=>array())));

Please anyone can help me !!!