proc_open()'s $env argument

proc_open_env.php

<?php
$fdSpecs = array(
    1 => array('pipe', 'w'),
    2 => array('pipe', 'w'),
);
$process = proc_open('php -r "var_dump(\$_ENV);"', $fdSpecs, $pipes, getcwd());
echo 'Without $env argument:', PHP_EOL;
echo stream_get_contents($pipes[1]);

echo PHP_EOL;

$process = proc_open('php -r "var_dump(\$_ENV);"', $fdSpecs, $pipes, getcwd(), NULL);
echo 'With NULL as $env argument:', PHP_EOL;
echo stream_get_contents($pipes[1]);

echo PHP_EOL;

$process = proc_open('php -r "var_dump(\$_ENV);"', $fdSpecs, $pipes, getcwd(), $_ENV);
echo 'With $_ENV as $env argument:', PHP_EOL;
echo stream_get_contents($pipes[1]);

echo PHP_EOL;

$process = proc_open('php -r "var_dump(\$_ENV);"', $fdSpecs, $pipes, getcwd(), array('foo' => 'bar'));
echo 'With additional $env argument:', PHP_EOL;
echo stream_get_contents($pipes[1]);

result.txt

$ php proc_open_env.php 
Without $env argument:
array(0) {
}

With NULL as $env argument:
array(0) {
}

With $_ENV as $env argument:
array(4) {
  ["PWD"]=>
  string(29) "/Users/yuya/dev/php/proc_open"
  ["SHLVL"]=>
  string(1) "1"
  ["_"]=>
  string(12) "/usr/bin/php"
  ["__CF_USER_TEXT_ENCODING"]=>
  string(10) "0x1F5:1:14"
}

With additional $env argument:
array(5) {
  ["PWD"]=>
  string(29) "/Users/yuya/dev/php/proc_open"
  ["foo"]=>
  string(3) "bar"
  ["SHLVL"]=>
  string(1) "1"
  ["_"]=>
  string(12) "/usr/bin/php"
  ["__CF_USER_TEXT_ENCODING"]=>
  string(10) "0x1F5:1:14"
}