Learning PHP was on my list of things to learn for a while. Two years ago, I finally started.
I learned PHP by taking two online courses at ed2go.com. They were called: Introduction to PHP and MySQL and Intermediate PHP and MySQL. They were fairly good at getting you started. The first course guides you through the creation of a recipe website with it’s own content management system (CMS). The second course guides you through the creation of an online store.
I also purchased a few books:
Codeacademy.com offers free online courses, including PHP. They are actually pretty good.
Php.net is the official PHP documentation site.
If you have the foundation of knowing another programming language, PHP is pretty easy to learn. For me, the hardest part is remembering to include the $ sign.