讨论Lumen / Laravel .env文件中环境变量的生效

　　.env 文件可自定义其他任何有效的环境变量，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，.env文件中的环境变量为何可以通过getenv()来获取呢？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 ： 。

在vlucas/phpdotenv/src/Loader.php文件中，我们可以看到.env被加载进一个数组，然后把每一行看作setter并调用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) 的定义如下：

/**
     * 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 itif (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

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