http://www.tizag.com/phpT/ is one of the best aswell as w3school but to keep advancing u have to use php.net
You can use PHP.net to begin to learn PHP, but your main resource would have to be online tutorials. When I first began, I had books, but it's also good to read free e-Books, or even online tutorials, they all help you.
I actually started off on a book called "PHP and MySQL Web Development" which to me was very packed. Course at the time I was reading it, PHP5 was only mere experimental mention and the current version was in the 4.0.* stage. (I think 4th edition covers php5 and the upcoming version 6). Course I had an advantage on my end, I knew C++, Pascal, VC++, ASP, Visual Basic, SQL Server and Oracle prior to going to PHP/MySQL route. So as a result I already had the basic programming knowledge and it was mainly a matter of learning the syntax and limitations. Sometimes the best way to learn is to simply try to create a web-app and learn from all the mistakes you make along the way. Just don't try to do so on a project you intend to put public on a short notice.
I think http://www.tizag.com/phpT/ is the best tutorial for beginners. If you want to know more about advanced features and functions than php.net is your second option.
I learned all the basics from online tutorials and only later started purchasing books on the subject (for more advanced topics of the language). I'd start with the basic tutorials!
Here is how I learned PHP, take pre-made scripts and modify them do something that suits your purpose.