This chapter covered the more advanced object-oriented features of PHP, many of which are critical when implementing large-scale OO applications. Thanks to the advances of PHP 5, using common OO methodologies, such as design patterns, has now become more of a reality than with past PHP versions. For further reading, we recommend additional material on design patterns and OO methodology. A good starting point is www.cetus-links.org, which keeps an up-to-date list of good starting points. Also, we highly recommend reading the classic book Design Patterns: Elements of Reusable Object-Oriented Software by Erich Gamma, Richard Helm, Ralph Johnson, and John M. Vlissides.