spl_autoload_register() just with added PSR-0 support

spl_autoload_register_psr0.php

<?php
/**
 * spl_autoload_register() just with added PSR-0 support
 *
 * The support is added, that means, for each classname first of all
 * by the priority given in the include-path directive, a per-directory
 * test is done for the full-classname based file and then sub-directories
 * are traversed.
 *
 * PHP 5.3 Namespace Separators are always mapped to a directory to
 * prevent different behavior on file-systems that are supporting the
 * backslash "\" like windows.
 *
 * @link http://www.php.net/manual/en/function.spl-autoload.php
 *
 * @param null $extensions
 *
 * @return bool
 */
$spl_autoload_register_psr0 = function ($extensions = null) {

    $callback = function ($className, $extensions = null) {

        if (!preg_match('(^(?:\\\\?[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*)+$)', $className)) {
            return;
        }


        null !== $extensions || $extensions = spl_autoload_extensions();
        $includePath = get_include_path();
        $extensions  = array_map('trim', explode(',', $extensions));
        $dirs        = array_map('realpath', explode(PATH_SEPARATOR, $includePath));

        $includes = array();

        $classStubNewNs = strtr($className, '\\', '/');
        $classStubOldNs = strtr($classStubNewNs, '_', '/');

        foreach ($dirs as $dir) {
            foreach ($extensions as $extension) {
                $includes[] = sprintf('%s/%s%s', $dir, $classStubNewNs, $extension);
                $includes[] = sprintf('%s/%s%s', $dir, $classStubOldNs, $extension);
            }
        }

        foreach ($includes as $file) {
            if (!is_readable($file)) {
                continue;
            }
            include $file;
        }
    };

    return spl_autoload_register($callback);
};