Firestore/src/DocumentReference.php
public function collection($collectionId)
    {
        return new CollectionReference(
            $this->connection,
            $this->valueMapper,
            $this->childPath($this->name, $collectionId)
        );
    }
public function collection($collectionId)
    {
        return new CollectionReference(
            $this->connection,
            $this->valueMapper,
            $this->childPath($this->name, $collectionId)
        );
    }
public function collection ( $ collectionId ) { return new CollectionReference ( $ this -> connection , $ this -> valueMapper , $ this -> childPath ( $ this -> name , $ collectionId ) ) ; }
Get a reference to a collection which is a child of the current document.

Example:
```
$child = $document->collection('wallet');
```

@param string $collectionId The ID of the child collection.
@return CollectionReference
Get a reference to a collection which is a child of the current document .