For complex web development, we make heavy use of PHP. Theres an embarrassment of riches as far as PHP books and resources go.
This is the official PHP project website, home to tutorials, to the annotated online manual (a great resource), the PHP software itself, and links to many other useful sites and resources.
Zend is a commercial entity, founded by some of the core authors and developers of PHP, that sells a number of tools for working with and enhancing PHP.
For books, you might look at Learning PHP 5 by David Sklar (ISBN: 0-59600-560-1). As your learning progresses, youll definitely want to look at Advanced PHP Programming by George Schlossnagle (ISBN: 0-67232-561-6).