can't set dynamic classname in php - php

Question

I have this function:
public function __construct()
{
$config = include APPPATH."email_config.php";
$driver = "Email_".ucfirst(strtolower($config['driver'])); echo $driver.RT; //gives Email_Smtp
// return new $driver($config); //do not work
return new Email_Smtp($config); //works
}
new $driver($config) gives the error message:
Fatal error: Uncaught Error: Class 'Email_Smtp' not found
$driver is the string "Email_Smtp".
return new Email_Smtp($config) works like a charm.
Whats wrong here?
the config file:
return array(
/**
* Mail useragent string wird in Header als X-Mailer: gesetzt
*/
'useragent' => 'my agent',
/**
* Mail driver (mail, smtp, sendmail, noop, php)
*/
'driver' => 'smtp',
...
here is my autoload:
class Autoloader {
/**
* #type string
*/
protected $namespacePrefix = '';
/**
* #type string
*/
protected $baseDir = '';
/**
* Sets the namespace's prefix.
* Only classes with this namespace will be autoloaded.
*
* #param string $prefix
* #return \Autoloader
*/
public function setNamespacePrefix($prefix) {
$this->namespacePrefix = $prefix;
return $this;
}
/**
* Sets the base directory, where we can find our php files.
*
* #param string $dir
* #return \Autoloader
*/
public function setBaseDir($dir) {
$this->baseDir = $dir;
return $this;
}
/**
* Register our autoloader.
*
* #return void
*/
public function register() {
spl_autoload_register(function($class) {
if (!preg_match('/^' . preg_quote($this->namespacePrefix) . '/', $class)) {
// Don't register a class without our namespace's prefix
return;
}
$relativeClass = substr($class, strlen($this->namespacePrefix));
$file = $this->baseDir . str_replace('\\', DIRECTORY_SEPARATOR, $relativeClass) . '.php';
if (file_exists($file)) {
require_once($file);
// if the loaded file contains a class...
if (class_exists($class, false))
{
if (method_exists($class, '_init') and is_callable($class.'::_init'))
{
call_user_func($class.'::_init');
}
}
}
});
}
}