<?php
// +----------------------------------------------------------------------
// | ThinkPHP [ WE CAN DO IT JUST THINK ]
// +----------------------------------------------------------------------
// | Copyright (c) 2006-2019 http://thinkphp.cn All rights reserved.
// +----------------------------------------------------------------------
// | Licensed ( http://www.apache.org/licenses/LICENSE-2.0 )
// +----------------------------------------------------------------------
// | Author: liu21st <liu21st@gmail.com>
// +----------------------------------------------------------------------

// [ 应用入口文件 ]
namespace think;

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

// 执行HTTP应用并响应
$http = (new App())->http;

$response = $http->run();

$response->send();

$http->end($response);

第一步 引入

项目根目录/vendor/autoload.php

autoload文件如下

<?php

// autoload.php @generated by Composer

require_once __DIR__ . '/composer/autoload_real.php';

return ComposerAutoloaderInitb4bb23f5554b6a38d22fe61fc7fad9b3::getLoader();

由此可见，要引入autoload_real.php，然后执行 

 ComposerAutoloaderInitb4bb23f5554b6a38d22fe61fc7fad9b3类下的getLoader方法

    /**
     * @return \Composer\Autoload\ClassLoader
     */
    public static function getLoader()
    {
        if (null !== self::$loader) {
            return self::$loader;
        }
       
        require __DIR__ . '/platform_check.php';

        spl_autoload_register(array('ComposerAutoloaderInitb4bb23f5554b6a38d22fe61fc7fad9b3', 'loadClassLoader'), true, true);
        self::$loader = $loader = new \Composer\Autoload\ClassLoader();
        spl_autoload_unregister(array('ComposerAutoloaderInitb4bb23f5554b6a38d22fe61fc7fad9b3', 'loadClassLoader'));

        $useStaticLoader = PHP_VERSION_ID >= 50600 && !defined('HHVM_VERSION') && (!function_exists('zend_loader_file_encoded') || !zend_loader_file_encoded());
        if ($useStaticLoader) {
            require __DIR__ . '/autoload_static.php';

            call_user_func(\Composer\Autoload\ComposerStaticInitb4bb23f5554b6a38d22fe61fc7fad9b3::getInitializer($loader));
        } else {
            $map = require __DIR__ . '/autoload_namespaces.php';
            foreach ($map as $namespace => $path) {
                $loader->set($namespace, $path);
            }

            $map = require __DIR__ . '/autoload_psr4.php';
            foreach ($map as $namespace => $path) {
                $loader->setPsr4($namespace, $path);
            }

            $classMap = require __DIR__ . '/autoload_classmap.php';
            if ($classMap) {
                $loader->addClassMap($classMap);
            }
        }

        $loader->register(true);

        if ($useStaticLoader) {
            $includeFiles = Composer\Autoload\ComposerStaticInitb4bb23f5554b6a38d22fe61fc7fad9b3::$files;
        } else {
            $includeFiles = require __DIR__ . '/autoload_files.php';
        }
        foreach ($includeFiles as $fileIdentifier => $file) {
            composerRequireb4bb23f5554b6a38d22fe61fc7fad9b3($fileIdentifier, $file);
        }

        return $loader;
    }

在getLoader方法 首先判断存在不存在$loader静态属性,如果不存在，则去引入当前目录下的（项目根目录/vendor/composer）platform_check.php 文件
platform_check.php主要是为了检测当前运行PHP的版本。

<?php

// platform_check.php @generated by Composer

$issues = array();
 
if (!(PHP_VERSION_ID >= 70205)) {
    $issues[] = 'Your Composer dependencies require a PHP version ">= 7.2.5". You are running ' . PHP_VERSION . '.';
}

if ($issues) {
    if (!headers_sent()) {
        header('HTTP/1.1 500 Internal Server Error');
    }
    if (!ini_get('display_errors')) {
        if (PHP_SAPI === 'cli' || PHP_SAPI === 'phpdbg') {
            fwrite(STDERR, 'Composer detected issues in your platform:' . PHP_EOL.PHP_EOL . implode(PHP_EOL, $issues) . PHP_EOL.PHP_EOL);
        } elseif (!headers_sent()) {
            echo 'Composer detected issues in your platform:' . PHP_EOL.PHP_EOL . str_replace('You are running '.PHP_VERSION.'.', '', implode(PHP_EOL, $issues)) . PHP_EOL.PHP_EOL;
        }
    }
    trigger_error(
        'Composer detected issues in your platform: ' . implode(' ', $issues),
        E_USER_ERROR
    );
}

如果检测通过，则去执行spl_autoload_register()自动加载函数