What is difference between use env('APP_ENV'), config('app.env') or App::environment() to get app environment?

In Short & up-to-date 2020:

• use env() only in config files

• use App::environment() for checking the environment (APP_ENV in .env).

• use config('app.var') for all other env variables, ex. config('app.debug')

• create own config files for your own ENV variables. Example:
  In your .env:

  MY_VALUE=foo

example config app/myconfig.php

return [    'myvalue' => env('MY_VALUE', 'bar'), // 'bar' is default if MY_VALUE is missing in .env];

Access in your code:

config('myconfig.myvalue') // will result in 'foo'

Explanation & History:

I just felt over it. When you cache your config file, env() will (sometimes?) not work right. So what I found out:

1. Laravel recommends only to use env() within the config files. Use the config() helper in your code instead of env(). For example you can call config('app.env') in your code.
2. When you use php artisan config:cache all the configuration strings are cached by the framework and any changes you make to your .env file will not be active until you run the php artisan config:cache command again.

From here:https://laracasts.com/discuss/channels/general-discussion/env-not-reading-variables-sometimes

UPDATE:
env() calls work as long as you don't use php artisan config:cache. So it's very dangerous because it will often work while development but will fail on production. See upgrade guide: https://laravel.com/docs/5.2/upgrade#upgrade-5.2.0

Caching And Env

If you are using the config:cache command during deployment, you mustmake sure that you are only calling the env function from within yourconfiguration files, and not from anywhere else in your application.

If you are calling env from within your application, it is stronglyrecommended you add proper configuration values to your configurationfiles and call env from that location instead, allowing you to convertyour env calls to config calls.

UPDATE Laravel 5.6:
Laravel now recommends in its documentation to use

$environment = App::environment();// or check on an array of environments:if (App::environment(['local', 'staging'])) {    // The environment is either local OR staging...}

and describes that env() is just to retrieve values from .env in config files, like config('app.env') or config('app.debug').

You have two equally good options

if (\App::environment('production')) {...}

or

if (app()->environment('production')) {...}

app()->environment() is actually used by Bugsnag, look in documentation here it says

By default, we’ll automatically detect the app environment by calling the environment() function on Laravel’s application instance.

Now, differences:

1) env(...) function returns null after caching config. It happens on production a lot.

2) you can change config parameters inside unit tests, it gives you flexibility while testing.

One thing to consider is perhaps the convenience factor of passing string to app()->environment() in order validate your current environment.

// or App:: whichever you prefer.if (app()->environment('local', 'staging')) {    logger("We are not live yet!");    Seeder::seedThemAll();} else {    logger("We are LIVE!");}