There is a tremendous amount of PHP reference material online. With everything from the annotated PHP manual to sites with periodic articles and tutorials, a fast Internet connection rivals a large bookshelf in PHP documentary usefulness. Here are some key sites:

The Annotated PHP Manual: http://www.php.net/manual/

Available in seventeen languages, this includes both official documentation of functions and language features as well as user-contributed comments.

PHP mailing lists: http://www.php.net/mailing-lists.php

There are many PHP mailing lists covering installation, programming, extending PHP, and various other topics. A read-only web interface to the mailing lists is at http://news.php.net/.

PHP Presentation archive: http://conf.php.net/

A collection of presentations on PHP given at various conferences.

PEAR: http://pear.php.net/

PEAR calls itself "a framework and distribution system for reuseable PHP components." You'll find lots of useful PHP classes and sample code there.

PHP.net: A Tourist's Guide: http://www.php.net/sites.php

This is a guide to the various web sites under the php.net umbrella.

PHP Knowledge Base: http://php.faqts.com/

Many questions and answers from the PHP community, as well as links to other resources.

PHP DevCenter: http://www.onlamp.com/php/

A collection of PHP articles and tutorials with a good mix of introductory and advanced topics.

• Programming PHP by Kevin Tatroe and Rasmus Lerdorf (O'Reilly).

• HTML and XHTML: The Definitive Guide by Chuck Musciano and Bill Kennedy (O'Reilly).

• Dynamic HTML: The Definitive Guide by Danny Goodman (O'Reilly).

• Mastering Regular Expressions by Jeffrey E. F. Friedl (O'Reilly).

• XML in a Nutshell by Elliotte Rusty Harold and W. Scott Means (O'Reilly).

• MySQL Reference Manual, by Michael "Monty" Widenius, David Axmark, and MySQL AB (O'Reilly); also available at http://www.mysql.com/documentation/.

• MySQL, by Paul DuBois (New Riders).

• Web Security, Privacy, and Commerce by Simson Garfinkel and Gene Spafford (O'Reilly).

• Web Services Essentials, by Ethan Cerami (O'Reilly).

• HTTP Pocket Reference, by Clinton Wong (O'Reilly).

• The Practice of Programming, by Brian W. Kernighan and Rob Pike (Addison-Wesley).

• Programming Pearls by Jon Louis Bentley (Addison-Wesley).

• The Mythical Man-Month, by Frederick P. Brooks (Addison-Wesley).