W3Schools is one of the best online tutorial sites available, but in addition to that, the site is completely free. When I can't remember how a particular bit of syntax goes I can be pretty much certain that I will find it on this site. There are thousands of examples, explanations, quizzes and reference charts available and they don't bog you down in too much detail. The examples on the site tell you exactly what you need to know about the subject so that you can copy the code and get on with the task at hand.
When writing PHP I usually use the www.php.net site as this has more detailed information, but it is still an excelent resource for beginners of PHP.
Finally, although it is easy to think that W3Schools is run by the W3C, they completely independent.