PSR-0和PSR-4自动加载机制及composer自动加载

1. 区别主要为：PSR0支持下划线（为了兼容5.2以前的第三方包，当时还没有命名空间，都是以下划线来区分的），psr0中类名的下划线将会转化为目录层级和相应的命名空间，如一个叫Swift_mail的类名存在于./Swift/Mail.php文件中，并且命名空间为当前命名空间\Swift.
2. 自动加载机制用的最多的便是composer。下面以workman为例，详解composer的使用方式：
   • 在项目目录下执行composer require workerman/workerman
   • vendor目录结构如下图：

vendor/
├── autoload.php
├── composer
│   ├── autoload_classmap.php
│   ├── autoload_namespaces.php
│   ├── autoload_psr4.php
│   ├── autoload_real.php
│   ├── ClassLoader.php
│   ├── installed.json
│   └── LICENSE
└── workerman
    └── workerman
        ├── Autoloader.php
        ├── composer.json
        ├── Connection
        │   ├── AsyncTcpConnection.php
        │   ├── ConnectionInterface.php
        │   ├── TcpConnection.php
        │   └── UdpConnection.php
        ├── Events
        │   ├── EventInterface.php
        │   ├── Event.php
        │   ├── Ev.php
        │   ├── Libevent.php
        │   └── Select.php
        ├── Lib
        │   ├── Constants.php
        │   └── Timer.php
        ├── MIT-LICENSE.txt
        ├── Protocols
        │   ├── Frame.php
        │   ├── Http
        │   │   └── mime.types
        │   ├── Http.php
        │   ├── ProtocolInterface.php
        │   ├── Text.php
        │   ├── Websocket.php
        │   └── Ws.php
        ├── README.md
        ├── WebServer.php
        └── Worker.php

• 对应的composer.json文件为：
  {
  "name" : "workerman/workerman",
  "type" : "project",
  "keywords": ["event-loop", "asynchronous"],
  "homepage": "http://www.workerman.net",
  "license" : "MIT",
  "description": "An asynchronous event driven PHP framework for easily building fast, scalable network applications.",
  "authors" : [
  {
  "name" : "walkor",
  "email" : "walkor@workerman.net",
  "homepage" : "http://www.workerman.net",
  "role": "Developer"
  }
  ],
  "support" : {
  "email" : "walkor@workerman.net",
  "issues": "https://github.com/walkor/workerman/issues",
  "forum" : "http://wenda.workerman.net/",
  "wiki" : "http://doc3.workerman.net/index.html",
  "source": "https://github.com/walkor/workerman"
  },
  "require": {
  "php": ">=5.3"
  },
  "suggest": {
  "ext-libevent": "For better performance."
  },
  "autoload": {
  "psr-4": {"Workerman\": "./"}
  },
  "minimum-stability":"dev"
  }

可以看出，vendor目录下面第一级为项目名workman/workman，在这层目录下面才是autoload里面注册的自动加载规则

3. composer.json中的autoload四种方式介绍

• classmap
  { "classmap": ["src/"]}，会将src目录下的所有文件遍历生成map到vendor/composer/autoload_classmap.php中去
• file（加载全局helper function）
  { "files": [ "path/to/file.php" ]}
• psr-0
• psr-4

http://feiyang.me/2012/05/php-psr-0-a-naming-standard/
https://psr.phphub.org/