Dan Olsen links to some Web-based tutorials, documentation, and books he's found useful in learning PHP.
Also, Jeremy Zawodny is arguing against abstraction layers in PHP, something that seems right to me. When I first read the title, I thought of Hibernate and other object-to-relational mapping tools. I'd argue that those abstraction layers are heaven sent. I don't think Jeremy would disagree since ultimately his argument comes down to the fact that PHP data abstraction layers don't really offer meaningful or useful abstractions--something that's not true of object-to-relational mapping layers.