In order to get all the documents in the collection, we will use MongoCollection::find(). The find() method returns a MongoCursor object which allows us to iterate over the set of documents that matched our query. So to query all of the documents and print them out:
$connection = new MongoClient();
$collection = $connection->database->collectionName;
$cursor = $collection->find();
foreach ( $cursor as $id => $value )
echo "$id: ";
var_dump( $value );
The API documentation on MongoCollection::find() contains more information about finding data.