Replace public $id property with getter method

src/GridFS/Bucket.php

@@ -183,8 +183,8 @@ class Bucket
     {
         $metadata = stream_get_meta_data($stream);
 
-        if (isset($metadata['wrapper_data']->id)) {
-            return $metadata['wrapper_data']->id;
+        if ($metadata['wrapper_data'] instanceof StreamWrapper) {
+            return $metadata['wrapper_data']->getId();
         }
 
         return;

src/GridFS/StreamWrapper.php

@@ -12,12 +12,17 @@ namespace MongoDB\GridFS;
 class StreamWrapper
 {
     public $context;
-    public $id;
 
     private $collectionsWrapper;
     private $gridFSStream;
+    private $id;
     private $mode;
 
+    public function getId()
+    {
+        return $this->id;
+    }
+
     public function openReadStream()
     {
         $context = stream_context_get_options($this->context);