Trouble Installing With Php 5.3

I am using PHP 5.3 with namespaces and am having difficulty installing. I have in my main index page the following:

spl_autoload_extensions(".php");
spl_autoload_register();

require_once 'htmlpurifier/library/HTMLPurifier.auto.php';
		$purifier_config=HTMLPurifier_Config::createDefault();
		$purifier_config->set('HTML.Allowed', 'p, em, b,a[href],i');
		$purifier = new HTMLPurifier($purifier_config);

This gives me the following error:

"Fatal error: Class 'themes\_default\forum\HTMLPurifier_Config' not found"

Is there a way to reference that class?

Please post your exact autoloader code. I made a test-case for what you described and it worked fine.

I use the default auto uploader: http://www.php.net/manual/en/function.spl-autoload.php

Yes, as I said, I made a test using the default autoloader, and it worked fine. I don't doubt your code is not working, so please post your exact PHP code.