Removed trailing whitespaces.
Enhanced the example for the Singleton design pattern.

- Moved static variable into the static creation method getInstance.
- Replaced "new self();" with "new static" (late static binding to allow
subclassing).
- Changed visibility of __construct to protected.
- Added the two private methods __clone and __wakeup.
- Added DocBlock comments.
- Updated description and enhanced with links to the PHP manual.