The most comprehensive tools for assessing websites are usually via a browser. There are a number of useful tools available as addons to popular browsers.
If you are serious about website optimizing you need to be running at least 3 browsers - a version of Internet Explorer (still probably the most popular), Firefox (No.2) and Google Chrome (No.3). It will help to show inconsistencies in your coding and you can see how Internet Explorer will often give you different results to Firefox and Chrome.
You can load multiple browsers and use them as required, without affecting performance of your PC.
An excellent addon for a range of browsers, I use Firefox, YSLOW
has been developed by Yahoo developers.
The Firefox Browser has an enormous range of addons for web developers. If you don't have Firefox you can download it HERE
Once you have Firefox, go to the downloads and make your choice from the wide range of developer tools - Yslow is amongst them, but there are others - Firebug is a good integrated tool, Colorzilla is useful for picking colours you like when surfing the internet, look through and see what you like.
The main website for the PHP language with details of functions, code and a host of useful information relating to PHP.
Interesting articles on a range of web related subjects, written by experts in their fields.
More articles and code snippets written by experts in their fields.
There are lots of blogs and other help sites. The best solution is to 'Google it' - if you have a problem put in the error message and you will likely find that there is someone who has asked the same question. Many problems can be solved this way.
There are also a lot of free resources on the internet. If you find a good one then bookmark it as there will often be other good pieces of information for the future. If you find a really good one, then other people know about it - that way the writer will benefit from increased traffic.