Sorry, but I'll disagree with w3schools. They have a pretty poor track record compared to php.net and other blogs/source. For more info, see http://w3fools.com/#wap
nettuts is a good resource (I've personally used one of their affiliate sites for extending my audio knowledge). I also recommend going to a Library and checking out a few PHP related books. Stay away from the PHP in 24 hour stuff, get a book that will take you the distance, but slowly, it is far more beneficial.