Directory

stdClass

__PHP_Incomplete_Class

exception

php_user_filter

Best way to check your reserved classes is,
var_dump (get_declared_classes ());

Standard Defined Classes

These classes are defined in the standard set of functions included in the PHP build.

Directory
The class from which dir is instantiated.
stdClass
Created by typecasting to object.
__PHP_Incomplete_Class
Possibly created by unserialize().

Predefined classes as of PHP 5

These additional predefined classes were introduced in PHP 5.0.0.

exception
php_user_filter

Closure

The predefined final class Closure was introduced in PHP 5.3.0. It is used for internal implementation of anonymous functions.

The class has a constructor forbidding the manual creation of the object (issues E_RECOVERABLE_ERROR) and the __invoke method with the calling magic.