定義一個常數，紀錄Laravel框架開始執行的時間。

這樣的常數通常是用來衡量框架本身的效能。

/*
|--------------------------------------------------------------------------
| Register The Composer Auto Loader
|--------------------------------------------------------------------------
|
| Composer provides a convenient, automatically generated class loader
| for our application. We just need to utilize it! We'll require it
| into the script here so that we do not have to worry about the
| loading of any our classes "manually". Feels great to relax.
|
*/

require __DIR__.'/../vendor/autoload.php';

真正的自動載入類別的功能，是由PHP的套件管理工具Composer來達成。
/*
|--------------------------------------------------------------------------
| Include The Compiled Class File
|--------------------------------------------------------------------------
|
| To dramatically increase your application's performance, you may use a
| compiled class file which contains all of the classes commonly used
| by a request. The Artisan "optimize" is used to create this file.
|
*/

if (file_exists($compiled = __DIR__.'/compiled.php'))
{
	require $compiled;
}

點開compiled.php來看，你會發現它內容超過1萬行。

Laravel的command line工具Artisan有optimize功能，能將常用到的類別全部放進這支compiled.php檔內。

不知道「常用到」是指哪些檔案呢？

/*
|--------------------------------------------------------------------------
| Setup Patchwork UTF-8 Handling
|--------------------------------------------------------------------------
|
| The Patchwork library provides solid handling of UTF-8 strings as well
| as provides replacements for all mb_* and iconv type functions that
| are not available by default in PHP. We'll setup this stuff here.
|
*/

Patchwork\Utf8\Bootup::initMbstring();

載入協助處理UTF-8字串的函式庫。
/*
|--------------------------------------------------------------------------
| Register The Laravel Auto Loader
|--------------------------------------------------------------------------
|
| We register an auto-loader "behind" the Composer loader that can load
| model classes on the fly, even if the autoload files have not been
| regenerated for the application. We'll add it to the stack here.
|
*/

Illuminate\Support\ClassLoader::register();

PHP在5.1.2版以後提供spl_autoload_register函式，能將某些類別先「登記」進一個queue內，之後使用該類別便不需要先載入、PHP會自動進這個queue內尋找載入該類別的方法。

上面已經由Composer負責「登記」大部份需要的系統類別了，而使用者自定義的類別(譬如說MVC的model檔案)，則在此處「登記」。
/*
|--------------------------------------------------------------------------
| Register The Workbench Loaders
|--------------------------------------------------------------------------
|
| The Laravel workbench provides a convenient place to develop packages
| when working locally. However we will need to load in the Composer
| auto-load files for the packages so that these can be used here.
|
*/

if (is_dir($workbench = __DIR__.'/../workbench'))
{
	Illuminate\Workbench\Starter::start($workbench);
}

Laravel提供了一個workbench類別，協助使用者在本機開發package。

不是說系統類別統一由一開始的Composer負責載入嗎？怎麼workbench會在這邊載入？

註解說明是因為workbench與Composer有相依性。
設計框架有一項困難之處就在這裡：框架用到的類別有一大堆，有自行開發的、有使用第3方函式庫的，該怎麼載入他們才漂亮？如何讓框架source code看起來不會亂七八糟？要全部在一個地方載入嗎？分散在不同地方會不會比較好？
附上PHP另一知名框架CodeIgniter負責核心載入的程式碼，供大家參考比較：
https://github.com/bcit-ci/CodeIgniter/blob/develop/system/core/CodeIgniter.php