For shame! How could you all forget php.net?
php.net contains wonderful documentation on php's features and functions, and is particularly useful if you've already got a head for programming. Check out
http://www.php.net/manual/en/ for the full documentation manual, but you can also just go
http://www.php.net/echo (or some other function name) to check out a particular function.
I would also recommend downloading either
wamp or
xamp. These are simple installers that install everything you need to get started with php (and perl if you use xampp). Most people give up learning php because you have to install a lot of stuff to get started.
All you have to do is place your php files inside of {xampp installation folder}/htdocs/ or {wamp installation folder}/www/, depending on if you installed wamp or xampp, then point your web browser to
http://localhost/your_php_file.php and you're steady away!
Cheers!