Personally when I need immediate help with a function, even though I have the PHP bible sitting on my desk in front of me, I will first turn to the online manual here:
http://us4.php.net/manual/en/
I do that primarily because of the comments below the base information provided by PHP and offer some great insight on what a particular function can do. With luck, if you are having a problem with a function, someone else will have had the same problem and had a solution proposed right there. Of course failing there, or from my books, www.google.com is my favorite as I have only rarely been able to find an answer to my questions on anything.