You can get more information about PHP, as well as the full PHP manual, at the PHP web site, http://www.php.net/. You can also subscribe to the PHP mailing list there. The PHP on-line manual is extremely useful: not only does it let you quickly look up any part of PHP, but it includes notes from people who use PHP about problems you might run into and how to fix them.
If you want more information about PHP in printed form, there is “Programming PHP” by Rasmus Lerdorf and Kevin Tatroe. Rasmus Lerdorf is one of the authors of the PHP language.
If you are interested in using PHP with the MySQL database there is also “Web Database Applications with PHP & MySQL” by Hugh E. Williams and David Lane, or “MySQL” by Paul DuBois. I’ve found the latter to be an extremely useful MySQL reference.