Setting Birthday variables to private, moving Birthday docs link to more appropriate location

Author: Nick Matthews

docs/GraphNode.fbmd

@@ -168,7 +168,7 @@ Returns the `link` property for the user as a string if present.
 ~~~~
 public \Facebook\GraphNodes\Birthday|null getBirthday()
 ~~~~
-Returns the `birthday` property for the user as a `\Facebook\GraphNodes\Birthday` if present.
+Returns the `birthday` property for the user as a [`Facebook\GraphNodes\Birthday`](/docs/php/Birthday) if present.
 
 ### getLocation() {#user-getlocation}
 ~~~~

docs/sdk_reference.fbmd

@@ -172,10 +172,6 @@ Graph nodes are collections that represent nodes returned by the Graph API. And
       '[`Facebook\\GraphNodes\\GraphUser`](/docs/php/GraphNode#user-instance-methods)',
       'A collection that represents a User node.',
     ],
-    [
-      '[`Facebook\\GraphNodes\\Birthday`](/docs/php/Birthday#instance-methods)',
-      'A class that represents a GraphUser\'s birthday.',
-    ],
   ],
 })
 </card>

src/Facebook/GraphNodes/Birthday.php

@@ -35,12 +35,12 @@ class Birthday extends DateTime
     /**
      * @var bool
      */
-    protected $hasDate = false;
+    private $hasDate = false;
 
     /**
      * @var bool
      */
-    protected $hasYear = false;
+    private $hasYear = false;
 
     /**
      * Parses Graph birthday format to set indication flags, possible values: