Contrary to what eng.mrkto.com said, getenv() isn't always case-insensitive. On Linux it is not:

<?php
var_dump(getenv('path')); // bool(false)
var_dump(getenv('Path')); // bool(false)
var_dump(getenv('PATH')); // string(13) "/usr/bin:/bin"

As noted on httpoxy.org, getenv() can confuse you in having you believe that all variables come from a "safe" environment (not all of them do).

In particular, $_SERVER['HTTP_PROXY'] (or its equivalent getenv('HTTP_PROXY')) can be manually set in the HTTP request header, so it should not be considered safe in a CGI environment.

In short, try to avoid using getenv('HTTP_PROXY') without properly filtering it.

All of the notes and examples so far have been strictly CGI.
It should not be understated the usefulness of getenv()/putenv() in CLI as well.

You can pass a number of variables to a CLI script via environment variables, either in Unix/Linux bash/sh with the "VAR='foo'; export $VAR" paradigm, or in Windows with the "set VAR='foo'" paradigm. (Csh users, you're on your own!) getenv("VAR") will retrieve that value from the environment.

We have a system by which we include a file full of putenv() statements storing configuration values that can apply to many different CLI PHP programs. But if we want to override these values, we can use the shell's (or calling application, such as ant) environment variable setting method to do so.

This saves us from having to manage an unmanageable amount of one-off configuration changes per execution via command line arguments; instead we just set the appropriate env var first.

This function is useful (compared to $_SERVER, $_ENV) because it searches $varname key in those array case-insensitive manner.
For example on Windows $_SERVER['Path'] is like you see Capitalized, not 'PATH' as you expected.
So just: <?php getenv('path') ?>

I did a benchmark about env.

constants : 
0.00067687034606934 ms
getenv : 
0.056761026382446 ms

(less is better)

https://github.com/eftec/php-benchmarks#define--const--env

And, in Windows at leat, reading the env value is considerably slow (in comparison with a constant), so PHP doesn't cache the information and asks to the OS the env value per call.

So, if you are calling once per request, then there is not a problem. However, if you are calling it many times per request, then it could affects the performance.

As you know, getenv('DOCUMENT_ROOT') is useful.
However, under CLI environment(I tend to do quick check
if it works or not), it doesn't work without modified php.ini
file. So I add "export DOCUMENT_ROOT=~" in my .bash_profile.

It is worth noting that since getenv('MY_VARIABLE') will return false when the variable given is not set, there is no direct way to distinguish between a variable that is unset and one that is explicitly set to the value bool(false) when using getenv().  
This makes it somewhat tricky to have boolean environment variables default to true if unset, which you can work around either by using "falsy" values such as 0 with the strict comparison operators or by using the superglobal arrays and isset().

for quick check of getenv() adding a new env variable -
if you add a new env variable, make sure not only apache but xampp is also restarted.
Otherwise getenv() will return false for the newly added env variable.

SERVER_NAME is the name defined in the apache configuration.
HTTP_HOST is the host header sent by the client when using the more recent versions of the http protocol.

Beware that when using this function with PHP built-in server – i.e. php -S localhost:8000 – it will return boolean FALSE.

From PHP 7.1 => getenv() no longer requires its parameter. If the parameter is omitted, then the current environment variables will be returned as an associative array.

Source: http://php.net/manual/en/migration71.changed-functions.php

When writing CLI applications, not that any environment variables that are set in your web server config will not be passed through.  PHP will pass through system environment variables that are prefixed based off the safe_mode_allowed_env_vars directive in your php.ini