To allow users to use BSON\Document instances like a standard PHP array or object, the class should implement functionality similar to ArrayAccess and magic accessors. This would allow the following uses:

// Before
$document->get('foo');
// After
$document['foo'];
$document->foo;

// Before
$document->has('foo');
// After
isset($document['foo']);
isset($document->foo);

Since instances of the class are a read-only, the following are expected to throw:

$document['foo'] = 'foo';
$document->foo = 'foo';
$document[] = 'foo';
unset($document['foo']);
unset($document->foo);

Accessing non-existing keys should throw the same way as Document::get does.