关于Lumen / Laravel .env 文件中的环境变量是如何生效的

　　.env 文件包含默认环境变量，我们还可自定义其他任何有效的变量，并可通过  调用 env() 或 $_SERVER 或 $_ENV  来获取该变量。那么env()是如何加载到这些变量的呢？在Lumen的vendor/laravel/lumen-framework/src/helpers.php中，我们可以发现env函数是这样被定义的：

if (! function_exists('env')) {
    /**
     * Gets the value of an environment variable. Supports boolean, empty and null.
     *
     * @param  string  $key
     * @param  mixed   $default
     * @return mixed
     */
    function env($key, $default = null)
    {
        $value = getenv($key);

        if ($value === false) {
            return value($default);
        }

        switch (strtolower($value)) {
            case 'true':
            case '(true)':
                return true;

            case 'false':
            case '(false)':
                return false;

            case 'empty':
            case '(empty)':
                return '';

            case 'null':
            case '(null)':
                return;
        }

        if (Str::startsWith($value, '"') && Str::endsWith($value, '"')) {
            return substr($value, 1, -1);
        }

        return $value;
    }
}

可见，env函数中调用了 getenv() 来读取环境变量。我们又知道getenv()是PHP原生提供可读取 $_SERVER 或 $_ENV 全局变量的函数API，那么问题来了，.env文件中的环境变量又是如何变成 $_SERVER 或 $_ENV 变量的一部分的呢？vlucas/phpdotenv 就是这个幕后功臣，在Lumen 或 Laravel 的vendor下可以找到她，如果要单独下载她，去这里 ：https://github.com/vlucas/phpdotenv 。

　　PHP dotenv 她是什么？

Loads environment variables from .env to getenv(), $_ENV and $_SERVER automagically.

This is a PHP version of the original Ruby dotenv.

　　Why .env?

You should never store sensitive credentials in your code. Storing configuration in the environment is one of the tenets of a twelve-factor app. Anything that is likely to change between deployment environments – such as database credentials or credentials for 3rd party services – should be extracted from the code into environment variables.

Basically, a .env file is an easy way to load custom configuration variables that your application needs without having to modify .htaccess files or Apache/nginx virtual hosts. This means you won't have to edit any files outside the project, and all the environment variables are always set no matter how you run your project - Apache, Nginx, CLI, and even PHP 5.4's built-in webserver. It's WAY easier than all the other ways you know of to set environment variables, and you're going to love it.

. NO editing virtual hosts in Apache or Nginx
. NO adding php_value flags to .htaccess files
. EASY portability and sharing of required ENV values
. COMPATIBLE with PHP's built-in web server and CLI runner