Do I need to use PHP autoload to use folders?

I am pretty confused about the implementation of the namespace in php, especially when it comes to aliases - importing classes.

I read a tutorial from this lesson:

• Using PHP V5.3 namespaces for readable and supported code (by Don Denoncourt, March 1, 2011, for IBM Developerworks)

But I don’t understand - when used __autoload, why should I store the alias classes in folders, but when not__autoload used , the alias in the namespace is just fine, as shown below,

<?php
namespace barbarian;
class Conan {
    var $bodyBuild = "extremely muscular";
    var $birthDate = 'before history';
    var $skill = 'fighting';
}
namespace obrien;
class Conan {
    var $bodyBuild = "very skinny";
    var $birthDate = '1963';
    var $skill = 'comedy';
}
use \barbarian\Conan as mother;
$conan = new mother();
var_dump($conan);
var_dump($conan->bodyBuild);

$conan = new \obrien\Conan();
var_dump($conan);
var_dump($conan->birthDate);
?>

Until I get an error, if I did not store Conan.phpin the folderbarbarian

<?php
require_once "autoload.php"; 
use \barbarian\Conan as Cimmerian;
$conan = new Cimmerian();
var_dump($conan);
?>

error message

: require (barbarian/Conan.php): : C:\wamp\www\test\2013\php\namepace\autoload.php 12

autoload.php:

<?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('\\', '/', $namespace) . '/';
  }
  $filename .= str_replace('_', '/', $classname) . '.php';
  require $filename;
}
?>

? , , autoload?