Autoloader

You should use an autoloader with Erfurt. It takes care of including the correct files:

set_include_path(
    get_include_path()
    . PATH_SEPARATOR . __DIR__ . '/lib/Erfurt/'
    . PATH_SEPARATOR . __DIR__ . '/lib/Zend/'
);
require_once 'Zend/Loader/Autoloader.php';
$al = Zend_Loader_Autoloader::getInstance();
$al->registerNamespace('Erfurt_');
$al->registerNamespace('Zend_');

In this example, the application file layout is as follows:

index.php  - the file in which the autoloader is setup
lib/
  Erfurt/  - cloned erfurt git repository
  Zend/    - "library/Zend" from the zend download