I've yet to find anything in a PHP book that I couldn't find online for free.
My favorite place for PHP info is php.net. Once you know the basic function names, you can jump right to their section by going to php.net/function_name. Ex: http://www.php.net/explode will take you right to the page for the explode() function.
Also, don't just read the main entries. There is a lot of good info in the user comments as well.
You can also download the whole manual, so you can access it on your computer if you're not online. I'd stick with the online version when connected, though--since it will always have the most recent comments.
Other good sources of PHP info:
Digital Point PHP Forum
DevShed PHP Forum
Sitepoint PHP Forum
WMW PHP Forum
And if you run into an error, doing a Google search for it in quotes will likely bring up a discussion where someone has already had/solved the same problem.
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