getenv() vs. $_ENV in PHP

According to the php documentation about getenv, they are exactly the same, except that getenv will look for the variable in a case-insensitive manner when running on case-insensitive file systems (like Windows). On Linux hosts it still works as case-sensitive. Most of the time it probably doesn't matter, but one of the comments on the documentation explains:

For example on Windows $_SERVER['Path'] is like you see, with the first letter capitalized, not 'PATH' as you might expect.

Because of that, I would probably opt to use getenv to improve cross-platform behavior, unless you are certain about the casing of the environment variable you are trying to retrieve.

Steve Clay's comment in this answer highlights another difference:

Added getenv() advantage: you don't need to check isset/empty before access. getenv() won't emit notices.

I know that the comment in the docs says that getenv is case-insensitive, but that's not the behaviour I'm seeing:

> env FOO=bar php -r 'print getenv("FOO") . "\n";'
bar
> env FOO=bar php -r 'print getenv("foo") . "\n";'

> env foo=bar php -r 'print getenv("foo") . "\n";'
bar
> env foo=bar php -r 'print getenv("FOO") . "\n";'

> php --version
PHP 5.4.24 (cli) (built: Jan 24 2014 03:51:25)
Copyright (c) 1997-2013 The PHP Group
Zend Engine v2.4.0, Copyright (c) 1998-2013 Zend Technologies

Looking at the source code for the getenv function, this is because there are three ways that PHP can fetch the environment variable:

1. Via sapi_getenv (e.g. if it's getting the environment variable from Apache)
2. If on Windows, from GetEnvironmentVariableA.
3. If on non-Windows, from the getenv function provided by libc.

As far as I can tell, the only time when it will behave in a case-insensitive manner is on Windows because that's how the Windows environment variable API behaves. If you're on Linux, BSD, Mac, etc then getenv is still case sensitive.

As mentioned by mario, $_ENV is not always populated due to different configurations of variables_order so it's best if you avoid $_ENV if you don't control the server configuration.

So, for the most portable PHP code:

1. Use getenv.
2. Use the correct case for the environment variable name.

Additionally $_ENV is typically empty if variables_order does't have E listed. On many setups it's likely that only $_SERVER is populated, and $_ENV is strictly for CLI usage.

On the other hand getenv() accesses the environment directly.

(Regarding the case-ambiguity, one could more simply employ array_change_key_case().)

I found getenv() useful to avoid a strange PHP bug where sometimes $_SERVER and $_ENV was undefined if auto_globals_jit was enabled (creating the _SERVER and _ENV variables when they're first used). Since then I began to to use it.