Here's a set of PHP specific books that I've found to be stepping stones. I was introduced to lots of very good concepts through them. The authors all have background and experience spanning various other languages and are quite aware of the various limitations of PHP, whether technical, in tools, or in its very watered down community.
Their love for the language doesn't cloud their judgment and they often offer some very constructive criticism that will raise your awareness about certain aspects of development in PHP compared to other communities. They may also offer alternatives to pass the limited PHP infrastructures when you want to practice in ways common to Java, .NET, Ruby or Python developers.
In no particular order:
- PHP Objects, Patterns and Practice
- Pro PHP: Patterns, Frameworks, Testing and More
- PHP in Action: Objects, Design, Agility
- Here's a free ebook, currently being written by Pádraic Brady: Survive The Deep End. At the time that I'm writing this, it only has 3 chapters and an appendix, but is already worth more than many PHP books that I've read. It's centered on the Zend Framework, but it seems to be more conceptual than technical, so I'm sure you can apply most of its content to many other frameworks. I would also recommend to check out Pádraic's blog. Lots of useful info in there.
Generally each author also has a blog, which links to other recommended resources, blogs, articles and books. That should keep you busy.
It took me some time to find these resources and after being aware of them, it made me realize how much time is spent looking for the signal through all the noise in the PHP community. I felt like just going Python or Ruby altogether, but I can't afford it right now. We need more developers like these guys, as they are the only thing that still validates PHP as a good candidate when looking for tools for serious professional development.