I think books is the best option to learn anything, I love having a book in my hand, eBooks are getting kinda okay as I have a SGT10.1, it doesn't feel like I am reading it as an eBook when I use it
I really like TheNewBoston and PhpAcademy youtube tutorials:
And the first place I learned php was at w3schools (however, I find it that it shouldn't be used anymore, check out w3fools)
Now I am using http://php.net/
as a ref. whenever I need help or've forgotten a code.