關於Lumen / Laravel .env 文件中的環境變數是如何生效的

　　.env 文件可自定義其他任何有效的環境變數，並可通過  調用 env() 或 $_SERVER 或 $_ENV  來獲取該變數。那麼env()是如何載入到這些變數的呢？在Lumen的vendor/laravel/lumen-framework/src/helpers.php中，我們可以發現env函數是這樣被定義的：

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;
    }
}

可見，env函數中調用了 getenv() 來讀取環境變數。我們又知道getenv()是PHP原生提供可讀取   $_SERVER 或 $_ENV   全局變數的函數API，.env文件中的環境變數為何可以通過getenv()來獲取呢？vlucas/phpdotenv 就是這個幕後功臣，在Lumen 或 Laravel 的vendor下可以找到她，如果要單獨下載她，去這裡 ：https://github.com/vlucas/phpdotenv 。

在vlucas/phpdotenv/src/Loader.php文件中，我們可以看到.env被載入進一個數組，然後把每一行看作setter並調用setEnvironmentVariable()方法：

/**
     * Load `.env` file in given directory.
     *
     * @return array
     */
    public function load()
    {
        $this->ensureFileIsReadable();

        $filePath = $this->filePath;
        $lines = $this->readLinesFromFile($filePath);
        foreach ($lines as $line) {
            if (!$this->isComment($line) && $this->looksLikeSetter($line)) {
                $this->setEnvironmentVariable($line);
            }
        }

        return $lines;
    }

/**
     * Read lines from the file, auto detecting line endings.
     *
     * @param string $filePath
     *
     * @return array
     */
    protected function readLinesFromFile($filePath)
    {
        // Read file into an array of lines with auto-detected line endings
        $autodetect = ini_get('auto_detect_line_endings');
        ini_set('auto_detect_line_endings', '1');
        $lines = file($filePath, FILE_IGNORE_NEW_LINES | FILE_SKIP_EMPTY_LINES);
        ini_set('auto_detect_line_endings', $autodetect);

        return $lines;
    }

 Loader::setEnvironmentVariable($name, $value = null) 的定義如下：

/**
     * Set an environment variable.
     *
     * This is done using:
     * - putenv,
     * - $_ENV,
     * - $_SERVER.
     *
     * The environment variable value is stripped of single and double quotes.
     *
     * @param string      $name
     * @param string|null $value
     *
     * @return void
     */
    public function setEnvironmentVariable($name, $value = null)
    {
        list($name, $value) = $this->normaliseEnvironmentVariable($name, $value);

        // Don't overwrite existing environment variables if we're immutable
        // Ruby's dotenv does this with `ENV[key] ||= value`.
        if ($this->immutable && $this->getEnvironmentVariable($name) !== null) {
            return;
        }

        // If PHP is running as an Apache module and an existing
        // Apache environment variable exists, overwrite it
        if (function_exists('apache_getenv') && function_exists('apache_setenv') && apache_getenv($name)) {
            apache_setenv($name, $value);
        }

        if (function_exists('putenv')) {
            putenv("$name=$value");
        }

        $_ENV[$name] = $value;
        $_SERVER[$name] = $value;
    }

　　PHP dotenv 她是什麼？

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