Having worked a great deal within the educational side of IT over the past 10 years, particularly in regards to PHP, this is a very interesting question. Everybody's preceding comments are indeed spot-on, however dagon's rather concise response of "Practice" really hit home.
There are quite a few great PHP books out there (I won't mention any in particular, as having written five and edited probably 15 others, I'm biased), but reading these alone and other learning resources aren't the final solution; they are tools which will help you learn in a fashion that counts the most: by building your own PHP-driven websites.
That said, I suggest just trying to build a website based around a topic that interests you; whether you want to build a simple e-commerce store, or your own photo album, it doesn't matter. Just try to build it. The lessons you'll learn along the way will prove to be invaluable. Also, keep a notepad nearby (virtual or otherwise) when writing the code, and take detailed notes regarding what you did wrong and how you fixed it.
Also, peruse great discussion boards such as this regularly, learning from the mistakes of others, and when you can, chime in with your own suggested answer.
Finally, choose a popular PHP-powered open source project, download the code, and read it!! You'll gain a tremendous amount of knowledge simply by reading the code of your peers.
Hope this helps, and best of luck with your learning!
Jason
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W. Jason Gilmore
http://www.easyphpwebsites.com/