If you're looking to learn more about PHP and related topics like databases, security, and XML, try starting with something from these lists.
Quite a slow read, but you will learn a lot from it despite it being relatively short.
Pitched at quite a high level, but it is the only book currently available that deals exclusively with making PHP work in highly scalable environments.
If you want a general introduction to the field of security, this is for you.
Highly recommended as a general introduction to Crypto topics.
An excellent all-around reference to database theory and SQL.
Soon to be released, but my copy is already on pre-order.
A long but worthwhile read that can take you quite far in the topic.
This is the easiest way to learn PHP 5 from scratch.
These are exceptionally comprehensive books and should really be on the bookshelves of all serious MySQL/PostgreSQL database adminstrators.
A bit out of date, but it's still an excellent, task-based reference.
This is highly technical, but fascinating, if you're looking to indoctrinate yourself in the security field.
Quite long and certainly not an exciting read in places, but fulfills its goal of being a comprehensive guide to security for Unix system administrators.
This book doesn't cover SVG. But if you want to know more about XML this is the first place to look.
The second volume is particularly of interest for more insight into randomization.
Kevin Mitnick is the ultimate bad guy turned good, and he approaches the topic of social engineering in an original and enlightening way.
Those looking to learn the fundamental principles of team management should look no further.
General Unix and C programming is very similar to PHP, so you can learn a lot about PHP by learning about the Unix shell.
The only book to buy if you want a stress-free guide to migrating from PHP 4 to 5.
A mixed bag of tricks for aspiring web developers.
Short and to the point, this is the quick fix guide to most XML problems.
The PHP manual is available from http://www.php.net/manual, and it is a consistently high-quality read.
Zend (http://www.zend.com) has a good set of PHP tutorials, and they also print various other popular editorials about the state of PHP.
PHP Builder (http://www.phpbuilder.com) publishes a number of high-quality PHP tutorials each year, and also has very active forums full of people ready to help.
DevShed (http://www.devshed.com) isn't as good as PHP Builder, but serves as a great backup resource if you have questions that aren't getting answered elsewhere.
Several application vendors try to boost their marketing efforts by offering PHP content. Oracle is perhaps the most prevalent, as it had several top PHP hackers write the Hitchhiker's Guide to PHP, available online for free at http://otn.oracle.com/pub/articles/php_experts. Similarly, IBM developerWorks has published a number of PHP tutorials at http://www-130.ibm.com/developerworks, some of which are actually good.
PEAR::DB has its own sets of documentation online, available at http://pear.php.net/manual/en/package.database.php. The database is thorough, if a little out of date now and then.
The online documentation for the SQLite library is at http://www.hwaci.com/sw/sqlite. I have found that it complements the PHP manual well.
All the content at http://www.cookiecentral.com. is available for free, and there is also an active messageboard for you to ask questions or see what others are saying.
There's a gentle (but quick) introduction to XPath at http://www.w3schools.com/xpath/default.asp.
To learn more about HTTP and protocols relating to it, the best and most authoritative source is the World Wide Web Consortium (W3C). You can view their HTTP information store online at http://www.w3.org/Protocols.
There are W3C specifications for XML, XPath, and XSLT online at http://www.w3.org/TR/2004/REC-xml-20040204, http://www.w3.org/TR/xpath, and http://www.w3.org/TR/xslt. They are quite dull and hard to understand—you have been warned!
Don't try to remember all the ASCII codes—you can find them online at http://www.asciitable.com.
Finally, if all else fails and you're still hunting around, you can visit my personal website at http://www.hudzilla.org, where I keep my own brand of PHP help.