Version 6, last updated by gnomeontherun at 25 Feb 00:35 UTC
1. Pro PHP: Patterns, Frameworks, Testing and More
By Kevin McArthur
Quite easy and readable. The second edition has some new chapters on Zend Framework, web2.0 etc
2. PHP Objects, Patterns, and Practice, Third Edition
By Matt Zandstra
Somewhat the same level as the book listed above. There's a good introduction on php5 OOP, it looks into a couple of basic patterns, and has a chapter on practices.
3. Head First Design Patterns
By Eric Freeman, Elisabeth Robson, Kathy Sierra, Bert Bates
The Head first series is quite different from any other book on development: it's full of fun examples, diagrams, and pictures of pretty girls. This book goes very deep but does so very gently. The code examples are java, but the concepts used in the book are mostly the same as in php, so that shouldn't scare anyone. Definitely a recommendation.
4. Design Patterns: Elements of Reusable Object-Oriented Software)
By Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides (aka Gang of Four or GoF)
The bible of design patterns. If you have a strong stomach
5. Patterns of Enterprise Application Architecture
By Martin Fowler
His site has lots of interesting information on patterns and application architecture as well.