<?php
/**
 * 一个具体项目的实例
 * 
 * 当使用 SPL 注册此自动加载器后，执行以下语句将从 
 * /path/to/project/src/Baz/Qux.php 载入 \Foo\Bar\Baz\Qux 类：
 * 
 *      new \Foo\Bar\Baz\Qux;
 *      
 * @param string $class 完整的类名
 * @return void
 */
spl_autoload_register(function ($class) {

    // 具体项目命名空间前缀
    $prefix = 'Foo\\Bar\\';

    // 命名空间前缀的基目录
    $base_dir = __DIR__ . '/src/';

    // 判断类名是否具有本命名空间前缀
    $len = strlen($prefix);
    if (strncmp($prefix, $class, $len) !== 0) {
        // 不含本命名空间前缀，退出本自动载入器
        return;
    }

    // 截取相应类名
    $relative_class = substr($class, $len);

    // 将命名空间前缀替作为文件基目录，然后
    // 将类名中的命名空间分隔符替换成文件分隔符,
    // 最后添加 .php 后缀
    $file = $base_dir . str_replace('\\', '/', $relative_class) . '.php';

    // 如果以上文件存在，则将其载入
    if (file_exists($file)) {
        require $file;
    }
});

<?php
namespace Example;

/**
 * An example of a general-purpose implementation that includes the optional
 * functionality of allowing multiple base directories for a single namespace
 * prefix.
 * 
 * Given a foo-bar package of classes in the file system at the following
 * paths ...
 * 
 *     /path/to/packages/foo-bar/
 *         src/
 *             Baz.php             # Foo\Bar\Baz
 *             Qux/
 *                 Quux.php        # Foo\Bar\Qux\Quux
 *         tests/
 *             BazTest.php         # Foo\Bar\BazTest
 *             Qux/
 *                 QuuxTest.php    # Foo\Bar\Qux\QuuxTest
 * 
 * ... add the path to the class files for the \Foo\Bar\ namespace prefix
 * as follows:
 * 
 *      <?php
 *      // instantiate the loader
 *      $loader = new \Example\Psr4AutoloaderClass;
 *      
 *      // register the autoloader
 *      $loader->register();
 *      
 *      // register the base directories for the namespace prefix
 *      $loader->addNamespace('Foo\Bar', '/path/to/packages/foo-bar/src');
 *      $loader->addNamespace('Foo\Bar', '/path/to/packages/foo-bar/tests');
 * 
 * The following line would cause the autoloader to attempt to load the
 * \Foo\Bar\Qux\Quux class from /path/to/packages/foo-bar/src/Qux/Quux.php:
 * 
 *      <?php
 *      new \Foo\Bar\Qux\Quux;
 * 
 * 以下代码将由 /path/to/packages/foo-bar/tests/Qux/QuuxTest.php 
 * 载入 \Foo\Bar\Qux\QuuxTest 类
 * 
 *      <?php
 *      new \Foo\Bar\Qux\QuuxTest;
 */
class Psr4AutoloaderClass
{
    /**
     * An associative array where the key is a namespace prefix and the value
     * is an array of base directories for classes in that namespace.
     *
     * @var array
     */
    protected $prefixes = array();

    /**
     * 在 SPL 自动加载器栈中注册加载器
     * 
     * @return void
     */
    public function register()
    {
        spl_autoload_register(array($this, 'loadClass'));
    }

    /**
     * 添加命名空间前缀与文件基目录对
     *
     * @param string $prefix 命名空间前缀
     * @param string $base_dir 命名空间中类文件的基目录
     * @param bool $prepend 为 True 时，将基目录插到最前，这将让其作为第一个被搜索到，否则插到将最后。
     * @return void
     */
    public function addNamespace($prefix, $base_dir, $prepend = false)
    {
        // 规范化命名空间前缀
        $prefix = trim($prefix, '\\') . '\\';

        // 规范化文件基目录
        $base_dir = rtrim($base_dir, '/') . DIRECTORY_SEPARATOR;
        $base_dir = rtrim($base_dir, DIRECTORY_SEPARATOR) . '/';

        // 初始化命名空间前缀数组
        if (isset($this->prefixes[$prefix]) === false) {
            $this->prefixes[$prefix] = array();
        }

        // 将命名空间前缀与文件基目录对插入保存数组
        if ($prepend) {
            array_unshift($this->prefixes[$prefix], $base_dir);
        } else {
            array_push($this->prefixes[$prefix], $base_dir);
        }
    }

    /**
     * 由类名载入相应类文件
     *
     * @param string $class 完整的类名
     * @return mixed 成功载入则返回载入的文件名，否则返回布尔 false
     */
    public function loadClass($class)
    {
        // 当前命名空间前缀
        $prefix = $class;

        // work backwards through the namespace names of the fully-qualified
        // class name to find a mapped file name
        while (false !== $pos = strrpos($prefix, '\\')) {

            // retain the trailing namespace separator in the prefix
            $prefix = substr($class, 0, $pos + 1);

            // the rest is the relative class name
            $relative_class = substr($class, $pos + 1);

            // try to load a mapped file for the prefix and relative class
            $mapped_file = $this->loadMappedFile($prefix, $relative_class);
            if ($mapped_file) {
                return $mapped_file;
            }

            // remove the trailing namespace separator for the next iteration
            // of strrpos()
            $prefix = rtrim($prefix, '\\');   
        }

        // 找不到相应文件
        return false;
    }

    /**
     * Load the mapped file for a namespace prefix and relative class.
     * 
     * @param string $prefix The namespace prefix.
     * @param string $relative_class The relative class name.
     * @return mixed Boolean false if no mapped file can be loaded, or the
     * name of the mapped file that was loaded.
     */
    protected function loadMappedFile($prefix, $relative_class)
    {
        // are there any base directories for this namespace prefix?
        if (isset($this->prefixes[$prefix]) === false) {
            return false;
        }

        // look through base directories for this namespace prefix
        foreach ($this->prefixes[$prefix] as $base_dir) {

            // replace the namespace prefix with the base directory,
            // replace namespace separators with directory separators
            // in the relative class name, append with .php
            $file = $base_dir
                  . str_replace('\\', DIRECTORY_SEPARATOR, $relative_class)
                  . '.php';
            $file = $base_dir
                  . str_replace('\\', '/', $relative_class)
                  . '.php';

            // 当文件存在时，在入之
            if ($this->requireFile($file)) {
                // 完成载入
                return $file;
            }
        }

        // 找不到相应文件
        return false;
    }

    /**
     * 当文件存在，则从文件系统载入之
     * 
     * @param string $file 需要载入的文件
     * @return bool 当文件存在则为 True，否则为 false
     */
    protected function requireFile($file)
    {
        if (file_exists($file)) {
            require $file;
            return true;
        }
        return false;
    }
}

<?php
namespace Example\Tests;

class MockPsr4AutoloaderClass extends Psr4AutoloaderClass
{
    protected $files = array();

    public function setFiles(array $files)
    {
        $this->files = $files;
    }

    protected function requireFile($file)
    {
        return in_array($file, $this->files);
    }
}

class Psr4AutoloaderClassTest extends \PHPUnit_Framework_TestCase
{
    protected $loader;

    protected function setUp()
    {
        $this->loader = new MockPsr4AutoloaderClass;

        $this->loader->setFiles(array(
            '/vendor/foo.bar/src/ClassName.php',
            '/vendor/foo.bar/src/DoomClassName.php',
            '/vendor/foo.bar/tests/ClassNameTest.php',
            '/vendor/foo.bardoom/src/ClassName.php',
            '/vendor/foo.bar.baz.dib/src/ClassName.php',
            '/vendor/foo.bar.baz.dib.zim.gir/src/ClassName.php',
        ));

        $this->loader->addNamespace(
            'Foo\Bar',
            '/vendor/foo.bar/src'
        );

        $this->loader->addNamespace(
            'Foo\Bar',
            '/vendor/foo.bar/tests'
        );

        $this->loader->addNamespace(
            'Foo\BarDoom',
            '/vendor/foo.bardoom/src'
        );

        $this->loader->addNamespace(
            'Foo\Bar\Baz\Dib',
            '/vendor/foo.bar.baz.dib/src'
        );

        $this->loader->addNamespace(
            'Foo\Bar\Baz\Dib\Zim\Gir',
            '/vendor/foo.bar.baz.dib.zim.gir/src'
        );
    }

    public function testExistingFile()
    {
        $actual = $this->loader->loadClass('Foo\Bar\ClassName');
        $expect = '/vendor/foo.bar/src/ClassName.php';
        $this->assertSame($expect, $actual);

        $actual = $this->loader->loadClass('Foo\Bar\ClassNameTest');
        $expect = '/vendor/foo.bar/tests/ClassNameTest.php';
        $this->assertSame($expect, $actual);
    }

    public function testMissingFile()
    {
        $actual = $this->loader->loadClass('No_Vendor\No_Package\NoClass');
        $this->assertFalse($actual);
    }

    public function testDeepFile()
    {
        $actual = $this->loader->loadClass('Foo\Bar\Baz\Dib\Zim\Gir\ClassName');
        $expect = '/vendor/foo.bar.baz.dib.zim.gir/src/ClassName.php';
        $this->assertSame($expect, $actual);
    }

    public function testConfusion()
    {
        $actual = $this->loader->loadClass('Foo\Bar\DoomClassName');
        $expect = '/vendor/foo.bar/src/DoomClassName.php';
        $this->assertSame($expect, $actual);

        $actual = $this->loader->loadClass('Foo\BarDoom\ClassName');
        $expect = '/vendor/foo.bardoom/src/ClassName.php';
        $this->assertSame($expect, $actual);
    }
}