Furthermore, popular platforms like WordPress, Drupal and Joomla are all based on PHP. If you want to want to customize these applications, or build plugins, you'll need to know PHP. And though you don't have to build Facebook applications with PHP, it's helpful know it when developing on that platform anyway.
Here are some resources to help get you started.
Build Your Own Database Driven Web Site Using PHP & MySQL
Sitepoint's Build Your Own Database Driven Web Site Using PHP & MySQL by Kevin Yank is a lucid introduction to both PHP and MySQL for the non-programmer. The full book costs money, but Sitepoint offers the first 168 pages for free. You'll have to fork over your e-mail address to get it, though.
However, you can find much of the material on the Web in this series of tutorials by Yank.
Zend's PHP 101
Zend, makers of the popular Zend Engine back-end for PHP, offer one of the most popular tutorials for PHP beginners: PHP 101: PHP For the Absolute Beginner by Vikram Vaswani.
Webmonkey's PHP and MySQL Tutorials
Practical PHP Programming
Practical PHP by PHP in a Nutshell author Paul Hudson is a free e-book for PHP beginners. A programming background is helpful, but not necessary, for readers of this book.
PHP 5 Power Programming
PHP 5 Power Programming by Andi Gutmans, Stig Bakken and Derick Rethans is not a beginner's book, but we're including it here for those looking for a next step. It's available in print or as a free PDF download here.
When in doubt, you can always consult the PHP Manual.