$_ENV
The $_ENV variable is a special superglobal variable that is used to retrieve the environment values.
The $_ENV variable is an associative array where the keys are the global variables and the values are the corresponding values.
$_ENV is a superglobal: it is always available, in every scope.
Values inside $_ENV are always strings.
The values in the $_ENV are collected at the start of the script, and do not change during the lifetime of the script. To get an up-to-date value, the function getenv() is preferable.
The content of the $_ENV array varies from installation to installation: there is no default list of keys in that array. It entirely depends on the underlying system.
<?php
// Displays the name of the user
echo $_ENV['USER'];
print_r($_ENV);
// Results is similar to this
/**
Array
(
[TERM] => xterm
[PATH] => /usr/bin:/bin
[LANG] => C
[SHELL] => /bin/sh
[MAIL] => /var/mail/nobody
[LOGNAME] => nobody
[USER] => nobody
[HOME] => /tmp
)
*/
?>