<?php
$n = fwrite(fopen("php://memory", "r"), "hello");
var_dump($n); // => int(0) on PHP, int(5) on HHVM(v3.6.0)

/*
 hhvm --version
HipHop VM 3.6.0 (rel)
Compiler: tags/HHVM-3.6.0-0-g6ef13f20da20993dc8bab9eb103f73568618d3e8
Repo schema: 3ab6576af394a33abbb29796c652f06b1c7640ce
*/

/*
 php -v
PHP 5.6.5 (cli) (built: Jan 26 2015 00:26:47)
Copyright (c) 1997-2014 The PHP Group
Zend Engine v2.6.0, Copyright (c) 1998-2014 Zend Technologies
*/