apache.org. For your localhost server, which enables you to run PHP at home.
php.net. For PHP, obviously. The documentation kicks serious butt, and you can find things in the manual simply by adding the name of the thing you want to find to the end of the php.net (example: http://www.php.net/explode). Some things you'll be using a lot are the $_GET[], $_POST[] and $_REQUEST[] superglobals, mysql_query(), mysql_fetch_row(). Something I bet you'll love about PHP once you get into it is the string handling, array handling, the interesting GD image library wrappers, and the simple-to-declare-and-use variables.
mySQL.com. MySQL is the thing you'll want to use to get databases for PHP. It's recommended you check somewhere else like w3schools or something when you're first learning about queries, since MySQL's documentation tended to go a little far in my opinion when I started :/. Of particular interest, you'll want to learn about and SELECT, INSERT, DELETE, UPDATE, AUTO_INCREMENT, PRIMARY_KEY, LAST_INSERT_ID, and NOW().
phpMyAdmin.net. phpMyAdmin is a very nice MySQL database managing thing in php. Most webhosts put a phpMyAdmin folder up to access the database of your online site. Use phpMyAdmin to make tables and databases (when you have database-making privileges), and even export data, tables and databases (which I wish I knew earlier, because I didn't want to spend all that time, manually updating the online database after changing my localhost ;__;)
If you're making a site with dynamic content, you might also want to use Cascading Style Sheets to make page generation simpler. Read up <form>s and <input>s too!