You can get more information about PHP, as well as the full PHP manual, at the PHP web site, http://php.net/. The PHP on-line manual is extremely useful: not only does it let you quickly look up any part of PHP, it also includes notes from people who use PHP about problems you might run into and how to fix them. When you’re looking at the official instructions for a function you’re having trouble with, do a search through the comments at the bottom of the page. Chances are, someone else has run into the same problem and has already posted a solution.
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, and MySQL by Paul DuBois. I’ve found the latter to be an extremely useful MySQL reference. You might also find my own MySQL tutorial at http://hoboes.com/NetLife/MySQL/ useful.
