<?php

$descriptor_spec = [
    0=> [ "pipe", "r" ],  // stdin is a pipe that the child will read from
    1=> [ "pipe", "w" ],  // stdout is a pipe that the child will write to
    2=> [ "pipe", "w" ],  // stderr is a file to write to
];

// NOTE: the array value is iterated by reference!
foreach ( $descriptor_spec as $fd => &$d )
{
    // don't do anything, just the fact that we used "&$d" will sink the ship!
    ;
    // my use case was validation of the array and adjusting some default values
}

$proc = proc_open( "/bin/true", $descriptor_spec, $pipes );
echo $proc === false ? "FAILED\n" : "SUCCEEDED\n";