include_path前提のnamespace対応autoload実装

autoload.php

<?php
namespace Acme;

set_include_path(implode(PATH_SEPARATOR, array(
    '/path/to/pear',
    '/path/to/vendor',
    get_include_path(),
)));

spl_autoload_register(function($className) {
    $className = ltrim($className, '\\');
    if (false !== ($pos = strrpos($className, '\\'))) {
        $namespace = substr($className, 0, $pos);
        $className = substr($className, $pos + 1);
        $fileName = str_replace('\\', DIRECTORY_SEPARATOR, $namespace)
            . DIRECTORY_SEPARATOR
            . str_replace('_', DIRECTORY_SEPARATOR, $className);
    } else {
        $fileName = str_replace('_', DIRECTORY_SEPARATOR, $className);
    }
    if (false !== ($path = stream_resolve_include_path($fileName . '.php'))) {
        return include $path;
    }
    return false;
}, true, true);

$e = new \PEAR_Exception('GRRRR');
$k1 = new \Acme\Klass();
$k2 = new Klass();

var_dump(get_class($e));// string(14) "PEAR_Exception"
var_dump(get_class($k1));// string(10) "Acme\Klass" 
var_dump(get_class($k2));// string(10) "Acme\Klass" 

名前空間のクラスとPEAR形式のクラスが併設されている時に発生するエラーの検証コードはこっち
https://gist.github.com/1680669