PHP Books and Sample Chapters

PHP Essentials

This looks like a fairly comprehensive free php book in wiki-style format. You can browse it freely online, but the downloadable pdf version is not free.

Programming : PHP at wikibooks

This is an open content php textbook from wikibooks. It covers the basic of the PHP language as well as some advanced concepts such as OOP and templating.

PHP 5 Power Programming

To download the book for free as a pdf file click on downloads under the More Information heading and a div will appear with the download link.

Practical PHP Programming

This is an unpublished book on PHP. This book assumes no PHP programming skill at all - you will be taught from scratch in that respect. It appears to be quite comprehensive and can be navigated online in tutorial fashion, so it serves as an online php book or php tutorial.

A Programmer's Introduction to PHP 4.0

Free PHP book online that can be downloaded in pdf format.

PHP Session Management With Cookies

Book excerpt at O'Reilly.