[5.2] Load custom .env file if it exists

[krzystof]

This allows to define several .env file, for example .env.testing. That means we could remove the configuration in phpunit,xml and move it to a custom env file.
If a file matching the APP_ENV exists, Laravel will load this file. Else, load the usual .env.

I have found it useful to run behat tests in the browser on a specific database connection (set the APP_ENV on a vhost).

[taylorotwell]

I guess I'm confused on how APP_ENV is even going to be set if we haven't loaded the default file yet?

[lucasmichot]

@taylorotwell APP_ENV can be already set as a system variable in php.ini, .profile or another file.
This variable will be set before loading the custom env file

@krzystof what if APP_ENV is set outside the code (eg. APP_ENV=testing) and the .env.testing contains APP_ENV=production ?
What about making APP_ENV immutable ?

[krzystof]

If an environment variable is already set, the Dotenv load calls does not overwrite existing values.

By default, Dotenv will NOT overwrite existing environment variables that are already set in the environment.

If you want Dotenv to overwrite existing environment variables, use overload instead of load

@taylorotwell In phpunit.xml for example, the APP_ENV is set to testing.

[krzystof]

@lucasmichot yes makes sense. Cleaner. I will commit this if @taylorotwell plans to merge this PR?

[taylorotwell]

Can you remove that unrelated test change?

[GrahamCampbell]

Please squash to one commit and rebase.

[lucasmichot]

@return void