come mai rievo quest omessaggio di errore?

Zend_Db_Adapter_Exception: The mysql driver is not currently installed



sotto il codice della pagina index.php

set_include_path(get_include_path().PATH_SEPARATOR .'D:/www/ZF-tutorial/library'.PATH_SEPARATOR.'.\models');

require_once('Zend/Loader.php');

Zend_Loader::registerAutoload();

try {
$db = Zend_Db::factory('Pdo_Mysql', array(
'host' => 'localhost',
'username' => 'Utente',
'password' => '********',
'dbname' => 'DBname'
));

$db->getConnection();
} catch (Zend_Db_Adapter_Exception $e) {
die("Zend_Db_Adapter_Exception: ".$e->getMessage());
} catch (Zend_Exception $e) {
die("Zend_Exception".$e->getMessage());
}