This class is used to insert an empty object into the database.
Inserting an empty array will create an array-type document, which can be used for (among other things) $push updates:
<?php
$collection->insert(array('x' => array()));
?>
The shell shows that an empty array is saved as an array:
> db.collection.findOne()
{ 'x' : [] }
Inserting a MongoEmptyObj will create an object-type document:
<?php
$collection->insert(array('x' => new MongoEmptyObj()));
?>
The shell shows that a MongoEmptyObj is saved as an object:
> db.collection.findOne()
{ 'x' : {} }