你如何通过php引用mongodb中的空集（{}）值字段？ [重复]

Question

This question already has an answer here:

• Best way to create an empty object in JSON with PHP? 6 answers
• How to write empty associative array ({}) to MongoDB from PHP 2 answers

I have the following code which works fine in my mongodb client:-

 db.whatevercollection.update({id:999999,external_ids:{}},
    {$unset: { "external_ids": true}})

How do I reference this empty set in php so that the external_ids field is completely removed? Using this:-

array('id' =>  $id,'external_ids'=>array())

Doesn't work...

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