HPHP/XHP: 'mixed' type in attribute declarations

We'd like to start using ##mixed## instead of ##var## for attribute types to be consistent with Hack. As a followup to this (once released), we would codemod all ##var## to ##mixed##.

hphp/runtime/base/string_buffer.cpp

@@ -3,14 +3,15 @@
    | HipHop for PHP                                                       |
    +----------------------------------------------------------------------+
    | Copyright (c) 2010-2013 Facebook, Inc. (http://www.facebook.com)     |
+   | Copyright (c) 1998-2010 Zend Technologies Ltd. (http://www.zend.com) |
    +----------------------------------------------------------------------+
-   | This source file is subject to version 3.01 of the PHP license,      |
+   | This source file is subject to version 2.00 of the Zend license,     |
    | that is bundled with this package in the file LICENSE, and is        |
    | available through the world-wide-web at the following url:           |
-   | http://www.php.net/license/3_01.txt                                  |
-   | If you did not receive a copy of the PHP license and are unable to   |
+   | http://www.zend.com/license/2_00.txt.                                |
+   | If you did not receive a copy of the Zend license and are unable to  |
    | obtain it through the world-wide-web, please send a note to          |
-   | license@php.net so we can mail you a copy immediately.               |
+   | license@zend.com so we can mail you a copy immediately.              |
    +----------------------------------------------------------------------+
 */