Books, magazines and online resources. Note: individual online articles are not referenced here, just sites with repositories of relevant info.
Bruce Eckel: Thinking in Java
Has a very solid lead-in to OOP which, although written for Java, can be mostly applied to PHP as well, making decent in depth discussion for beginners. Also nice is it’s free to read online or you can pick it up from Amazon here
Unfinished book on design patterns. Originally planned to use Java but most examples work with Python, bringing a dynamic aspect to it that’s relevant to PHP
GOF: Design Patterns
A must read. Has become a modern-day classic on software development - the missing link on OOP. Provides the core design patterns which can be applied in most any programming language.
Martin Fowler: Patterns of Enterprise Application Architecture
One of the “new wave” books on patterns, addressing issues directly relevant to web applications. Another must read
Martin Fowler: Refactoring: Improving the Design of Existing Code
A book that will improve a developer’s code quality faster than any other. Everything from how large methods should be to how to safely put right a completely mis-designed application.
BlueJ Editor and [url=http://www.bluej.org/objects-first/]Objects First
Java: learning by doing - principles can be applied to any language with an object model.
Provides a discussion of sixteen design patterns which are relevant to web application development, while threading refactoring and unit testing into the narrative. Uses both PHP4 and PHP5 as reference examples. A more in detailed review.
Head First Design Patterns
A different way of learning patterns, using a visually-rich formatted book. The book uses Java examples.
Excellent US-based PHP magazine bridging the gap between books and typical online articles - much depth while to the point. Distributed in electronic and print formats.
International PHP Magazine
Run out of Germany, this gets input from many of PHP’s core developers so has an inside track there but is still working to establish itself. Same company also produces the for-print German PHP Magazine (which has completely different content)
PHP Solutions Magazine
[small]Seen this in German on the local newsstands. Seems to be addressing the PHP hacker-ethic (and uses very bright colors on the cover).