first 2 years i never touched a book..just articles and php docs..later i solidified my learning with a few books
Books(and eBooks for that matter) are a great way to study, though, If you're more of a hands on and self-learning type of person or if you have any experience in other languages, it would be a lot easier to learn PHP using PHP.net's Documentation, i would download a copy and start reading through it, you'd be surprised how easy it is to learn through it. here is the link to the latest online version of the documentation in english: http://www.php.net/manual/en/
I learned PHP by reading tutorials and books, but before i learned php i already know some other programming languages that's why it's more quicker and easy for me to understand PHP. It took me 3 weeks to learn PHP. The only problem that you will encounter in learning PHP is the logic of the script or program that you are making.
The most important thing is the concepts and logics, these are the bread and butter of any programming language. you should know the basics well before moving on always.
By reading books, few tutorials and practice to make customized scripts took around 1 year to learn php but learning is endless process.. and i am free now ,looking for php work..
lots of book lots of tutorials and just downloading code and modifying it Ive been working with it about 5 years.
mostly palying around with scripts, then reading some quick tips from php.net but as i said mostly from practice, seing things in action and undestand what does what.
w3schools and video tutorials. it will help us alot. but after learning we need lots of practice. then only we can work it out.
By editing Wordpress themes, but I only picked up the basics doing that. I still have a long way to go!