It seems that stream_get_contents() on STDOUT blocks infinitly under Windows when STDERR is filled under some circumstances.
The trick is to open STDERR in append mode ("a"), then this will work, too.
<?php
$descriptorspec = array(
 0 => array('pipe', 'r'), // stdin
 1 => array('pipe', 'w'), // stdout
 2 => array('pipe', 'a') // stderr
);
?>