• 一个完全合格的namespace和class必须符合这样的结构 \<Vendor Name><Namespace>*<Class Name>
• 每个namespace必须有一个顶层的namespace（"Vendor Name"提供者名字）
• 每个namespace可以有多个子namespace
• 当从文件系统中加载时，每个namespace的分隔符要转换成 DIRECTORY_SEPARATOR(操作系统路径分隔符)
• 在CLASS NAME（类名）中，每个下划线(_)符号要转换成DIRECTORY_SEPARATOR。在namespace中，下划线(_)符号是没有（特殊）意义的。
• 当从文件系统中载入时，合格的namespace和class一定是以 .php 结尾的
• verdor name,namespaces,class名可以由大小写字母组合而成（大小写敏感的）

• \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

• \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

<?php
 
/*
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 * This software consists of voluntary contributions made by many individuals
 * and is licensed under the MIT license. For more information, see
 * <http://www.doctrine-project.org>.
 */
 
/**
 * SplClassLoader implementation that implements the technical interoperability
 * standards for PHP 5.3 namespaces and class names.
 *
 * http://groups.google.com/group/php-standards/web/psr-0-final-proposal?pli=1
 *
 *     // Example which loads classes for the Doctrine Common package in the
 *     // Doctrine\Common namespace.
 *     $classLoader = new SplClassLoader('Doctrine\Common', '/path/to/doctrine');
 *     $classLoader->register();
 *
 * @license http://www.opensource.org/licenses/mit-license.html  MIT License
 * @author Jonathan H. Wage <jonwage@gmail.com>
 * @author Roman S. Borschel <roman@code-factory.org>
 * @author Matthew Weier O'Phinney <matthew@zend.com>
 * @author Kris Wallsmith <kris.wallsmith@gmail.com>
 * @author Fabien Potencier <fabien.potencier@symfony-project.org>
 */
class SplClassLoader
{
    private $_fileExtension = '.php';
    private $_namespace;
    private $_includePath;
    private $_namespaceSeparator = '\\';
 
    /**
     * Creates a new <tt>SplClassLoader</tt> that loads classes of the
     * specified namespace.
     * 
     * @param string $ns The namespace to use.
     */
    public function __construct($ns = null, $includePath = null)
    {
        $this->_namespace = $ns;
        $this->_includePath = $includePath;
    }
 
    /**
     * Sets the namespace separator used by classes in the namespace of this class loader.
     * 
     * @param string $sep The separator to use.
     */
    public function setNamespaceSeparator($sep)
    {
        $this->_namespaceSeparator = $sep;
    }
 
    /**
     * Gets the namespace seperator used by classes in the namespace of this class loader.
     *
     * @return void
     */
    public function getNamespaceSeparator()
    {
        return $this->_namespaceSeparator;
    }
 
    /**
     * Sets the base include path for all class files in the namespace of this class loader.
     * 
     * @param string $includePath
     */
    public function setIncludePath($includePath)
    {
        $this->_includePath = $includePath;
    }
 
    /**
     * Gets the base include path for all class files in the namespace of this class loader.
     *
     * @return string $includePath
     */
    public function getIncludePath()
    {
        return $this->_includePath;
    }
 
    /**
     * Sets the file extension of class files in the namespace of this class loader.
     * 
     * @param string $fileExtension
     */
    public function setFileExtension($fileExtension)
    {
        $this->_fileExtension = $fileExtension;
    }
 
    /**
     * Gets the file extension of class files in the namespace of this class loader.
     *
     * @return string $fileExtension
     */
    public function getFileExtension()
    {
        return $this->_fileExtension;
    }
 
    /**
     * Installs this class loader on the SPL autoload stack.
     */
    public function register()
    {
        spl_autoload_register(array($this, 'loadClass'));
    }
 
    /**
     * Uninstalls this class loader from the SPL autoloader stack.
     */
    public function unregister()
    {
        spl_autoload_unregister(array($this, 'loadClass'));
    }
 
    /**
     * Loads the given class or interface.
     *
     * @param string $className The name of the class to load.
     * @return void
     */
    public function loadClass($className)
    {
        if (null === $this->_namespace || $this->_namespace.$this->_namespaceSeparator === substr($className, 0, strlen($this->_namespace.$this->_namespaceSeparator))) {
            $fileName = '';
            $namespace = '';
            if (false !== ($lastNsPos = strripos($className, $this->_namespaceSeparator))) {
                $namespace = substr($className, 0, $lastNsPos);
                $className = substr($className, $lastNsPos + 1);
                $fileName = str_replace($this->_namespaceSeparator, DIRECTORY_SEPARATOR, $namespace) . DIRECTORY_SEPARATOR;
            }
            $fileName .= str_replace('_', DIRECTORY_SEPARATOR, $className) . $this->_fileExtension;
 
            require ($this->_includePath !== null ? $this->_includePath . DIRECTORY_SEPARATOR : '') . $fileName;
        }
    }
}

看到这里你可能还有疑问，比如psr-0中给出的例子

\Symfony\Core\Request => /path/to/project/lib/vendor/Symfony/Core/Request.php

在=>号之前是我们要去加载的类，而后面的部分是这个文件在文件系统中的路径(我们实际加载的是这个)。在加载Symfony\Core\Request时，并没有同时给出实际路径来，那么是如何得到这个映射的呢？在PSR-0标准中并没有对/path/to/project/lib路径的强制要求，而是可以有自己的实现。一般有两种方式，一种就是使用php配置中的include_path（上面给出的例子有使用这个方式来做），第二种就是自己去注册这种映射，可以选用其中一种，或者两种都兼容。下面就来看看在Symfony中是怎么做的？

<?php
require_once '/path/to/src/Symfony/Component/ClassLoader/ClassLoader.php';

use Symfony\Component\ClassLoader\ClassLoader;

$loader = new ClassLoader();

// to enable searching the include path (eg. for PEAR packages)
$loader->setUseIncludePath(true);

// ... register namespaces and prefixes here - see below

$loader->register();

// register a single namespaces
$loader->addPrefix('Symfony', __DIR__.'/vendor/symfony/symfony/src');

此时就将 Symfony 这个namespace 注册到了一个真实的路径上，Symfony的子命名空间也可以使用这个路径了。

完。