This site! 🙂
seriously.. go through it.. LOTS of helpful articles etc.
Also.. php.net (obviously) better for looking up specific functions.
http://www.evilwalrus.com
http://php.resourceindex.com
etc etc.
I learnt by reading other php scripts..s tarting with basic ones to see how they did what they did.. then working my way up.. then using all the little bits of knowledge to plug together my own scripts.
My code is very messy and probably not the most efficient, but that's what i'm learning now thanks to 2 good books:
MySQL/PHP Database Applications (Jay Greenspan, Brad Bulger .. www.mandtbooks.com)
Core PHP Programming (Leon Atkinson)