Learning Help

Hi,

Over the last few months i have started mucking around build my website and the more and more i get involved with it the more i enjoy it.
but i am starting to want to learn more in depth.

I kinda know html, but not off by heart is still use some references.
CSS i am starting to get the hang of.
but now i would like to learn php and more in depth things.

I would like to be able to create a plugin in the future I live in the uk and course to learn such thing are well expensive.

So I am asking WordPress community if they can help me with useful websites, forums, books, magazines. that will help build my knowledge of the creation of web based material.

Thanks
Ricky