pipes opened by proc_open are not working under Linux (I have TSL 1.5 on Linux kernel 2.2.25).
I tryed the same code under FreeBSD and TSL and there was no success in the last case (it working good under FreeBSD). PHP doesn't report any error, but I have no data in the chlid process output stream.
php compiled with --with-mysql --with-apxs
Here is the sample that not working on my system:

$descriptorspec = array( 
  0 => array("pipe", "w"), // stdin
  1 => array("pipe", "r"), // stdout
  2 => array("pipe", "r")  // stderr
); 

$process = proc_open("cat", $descriptorspec, $pipes);
if (is_resource($process))
{ 

   fwrite($pipes[0], "Any string"); 
   fclose($pipes[0]); 

   while(!feof($pipes[1])) 
       echo fgets($pipes[1], 1024); 
   fclose($pipes[1]); 
   fclose($pipes[2]);
   $return_value = proc_close($process); 
}