More pipes?

Is there a way to open more pipes? In normal PHP I can use proc_open to do something like this:

$descriptors = array(
    0 => array("pipe", "r"),  // STDIN
    1 => array("pipe", "w"),  // STDOUT
    2 => array("pipe", "w"),  // STDERR
    3 => array("pipe", "w"),  // DATA
);
$proc = proc_open("data_handler", $descriptors, $pipes, "../../../submodules/data_handler/program");
foreach ($inputs as $input) 
    fwrite($pipes[0], $input.PHP_EOL);
fclose($pipes[0]);

$data   = stream_get_contents($pipes[3]); fclose($pipes[3]);
$stderr = stream_get_contents($pipes[2]); fclose($pipes[2]);
$stdout = stream_get_contents($pipes[1]); fclose($pipes[1]);
$exitCode = proc_close($proc);

I couldn't find any documentation on this.

Start by reading the process library's public API and documentation against the PHP proc_open descriptor example in the issue. Determine whether additional pipes are supported or would require a new API, then document the supported behavior or define the needed change and its completion criteria.