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The following describes the mandatory requirements that must be adhered to for autoloader interoperability.

Mandatory
A fully-qualified namespace and class must have the following structure \<Vendor Name>\(<Namespace>\)*<Class Name>
Each namespace must have a top-level namespace ("Vendor Name").
Each namespace can have as many sub-namespaces as it wishes.
Each namespace separator is converted to a DIRECTORY_SEPARATOR when loading from the file system.
Each _ character in the CLASS NAME is converted to a DIRECTORY_SEPARATOR. The _ character has no special meaning in the namespace.
The fully-qualified namespace and class is suffixed with .php when loading from the file system.
Alphabetic characters in vendor names, namespaces, and class names may be of any combination of lower case and upper case.
Examples
\Doctrine\Common\IsolatedClassLoader => /path/to/project/lib/vendor/Doctrine/Common/IsolatedClassLoader.php
\Symfony\Core\Request => /path/to/project/lib/vendor/Symfony/Core/Request.php
\Zend\Acl => /path/to/project/lib/vendor/Zend/Acl.php
\Zend\Mail\Message => /path/to/project/lib/vendor/Zend/Mail/Message.php
Underscores in Namespaces and Class Names
\namespace\package\Class_Name => /path/to/project/lib/vendor/namespace/package/Class/Name.php
\namespace\package_name\Class_Name => /path/to/project/lib/vendor/namespace/package_name/Class/Name.php
The standards we set here should be the lowest common denominator for painless autoloader interoperability. You can test that you are following these standards by utilizing this sample SplClassLoader implementation which is able to load PHP 5.3 classes.

Example Implementation
Below is an example function to simply demonstrate how the above proposed standards are autoloaded.

<?php

function autoload($className)
{
$className = ltrim($className, '\\');
$fileName = '';
$namespace = '';
if ($lastNsPos = strrpos($className, '\\')) {
$namespace = substr($className, 0, $lastNsPos);
$className = substr($className, $lastNsPos + 1);
$fileName = str_replace('\\', DIRECTORY_SEPARATOR, $namespace) . DIRECTORY_SEPARATOR;
}
$fileName .= str_replace('_', DIRECTORY_SEPARATOR, $className) . '.php';

require $fileName;
}
SplClassLoader Implementation
The following gist is a sample SplClassLoader implementation that can load your classes if you follow the autoloader interoperability standards proposed above. It is the current recommended way to load PHP 5.3 classes that follow these standards.

http://gist.github.com/221634

http://feiyang.me/2012/05/php-psr-0-a-n ... qus_thread

PSR是Proposing a Standards Recommendation（提出标准建议）的缩写，是由PHP Framework Interoperability Group（PHP通用性框架小组，简称PHP-FIG）发起的，通过他们命名就可以看出，这是个主要是针对框架通用性而做努力的开放性小组，他们的在Github上有自己的仓库地址，目前只有一个被接受的标准，那就是PSR-0标准，标准定义了PHP自动加载的命名规范和文件路径规范。 针对PSR-0标准主要提到了以下几点：
要求
一个完全合格的命名空间和类名必须有以下的结构“\<提供者名称>\(<命名空间>\)*<类名>”
每个命名空间必须有顶级的命名空间（“提供者”）
每个命名空间可以有任意多个子命名空间
每个命名空间在被从文件系统加载时必须被转换为“操作系统路径分隔符”（DIRECTORY_SEPARATOR ）
每个“_”字符在“类名”中被转换为DIRECTORY_SEPARATOR 。“_”符号在命名空间中没有这个含义
符合命名标准的命名空间和类名必须以“.php”结尾来加载文件
提供商名称，命名空间，类名可以由大小写字母组成，其中命名空间和类名是大小写敏感的以保证多系统兼容性
如果文件不存在需要返回false
例子
\Doctrine\Common\IsolatedClassLoader => /path/to/project/lib/vendor/Doctrine/Common/IsolatedClassLoader.php
\Symfony\Core\Request => /path/to/project/lib/vendor/Symfony/Core/Request.php
\Zend\Acl => /path/to/project/lib/vendor/Zend/Acl.php
\Zend\Mail\Message => /path/to/project/lib/vendor/Zend/Mail/Message.php
下划线在命名空间和类名中的使用
\namespace\package\Class_Name => /path/to/project/lib/vendor/namespace/package/Class/Name.php
\namespace\package_name\Class_Name => /path/to/project/lib/vendor/namespace/package_name/Class/Name.php
设置这个标准是为了保证最基本的共同点。你可以通过实现5.3的SplClassLoader来测试这个标准。
扩展例子
提供一个函数来展示如何使用上述标准。
<?php

function autoload($className)
{
$className = ltrim($className, '\\');
$fileName = '';
$namespace = '';
if ($lastNsPos = strripos($className, '\\')) {
$namespace = substr($className, 0, $lastNsPos);
$className = substr($className, $lastNsPos + 1);
$fileName = str_replace('\\', DIRECTORY_SEPARATOR, $namespace) . DIRECTORY_SEPARATOR;
}
$fileName .= str_replace('_', DIRECTORY_SEPARATOR, $className) . '.php';

require $fileName;
}
SplClassloader的实现
接下来这个gist实现了SplClassLoader可以加载你按照上面标准来实现的通用类库，这是5.3里面推荐的加载方式。
http://gist.github.com/221634
扩展实现
因为这个标准提到了如果文件不存在的时候应该范围false，但是在上面函数的例子中并没有实现该机制，所有有人实现了优化的SplClassLoader。
<?php

class ClassLoader
{
/**
* @var array Contains namespace/class prefix as key and sub path as value
*/
protected $paths;

/**
* Construct a loader instance
*
* @param array $paths Containing class/namespace prefix as key and sub path as value
*/
public function __construct( array $paths )
{
$this->paths = $paths;
}

/**
* Load classes/interfaces following PSR-0 naming
*
* @param string $className
* @return null|boolean Null if no match is found, bool if match and found/not found.
*/
public function load( $className )
{
if ( $className[0] === '\\' )
$className = substr( $className, 1 );

foreach ( $this->paths as $prefix => $subPath )
{
if ( strpos( $className, $prefix ) !== 0 )
continue;

$lastNsPos = strripos( $className, '\\' );
$prefixLen = strlen( $prefix ) + 1;
$fileName = $subPath . DIRECTORY_SEPARATOR;

if ( $lastNsPos > $prefixLen )
{
// Replacing '\' to '/' in namespace part
$fileName .= str_replace(
'\\',
DIRECTORY_SEPARATOR,
substr( $className, $prefixLen, $lastNsPos - $prefixLen )
) . DIRECTORY_SEPARATOR;
}

// Replacing '_' to '/' in className part and append '.php'
$fileName .= str_replace( '_', DIRECTORY_SEPARATOR, substr( $className, $lastNsPos + 1 ) ) . '.php';

if ( ( $fileName = stream_resolve_include_path( $fileName ) ) === false )
return false;

require $fileName;
return true;
}
}
}
引用地址：https://github.com/andrerom/fig-standar ... d/PSR-2.md
写在后面
标准对于开发者来说是一个好事，如今已经越来越多的开源项目加入了这个标准Pear2、PHPBB、Composer、Packagist、Joomla、Drupal、Symfony、CakePHP、Doctrine2等等，我常用的一个框架MicroMVC也在很早的时候就采用了这个标准。采用同样标准的项目可以无缝的接入，做为开发者最好要尝试并接收一个好的标准。
扩展阅读
PHP官方关于SplClassLoader的RFC：https://wiki.php.net/rfc/splclassloader PHP标准化组织论坛：https://groups.google.com/forum/?fromgr ... -standards