Some sites for theme/php learning?

Hello, I want to learn a bit about WP theme editing and php learning. Does anyone know a good site that I can learn from that explains most of how the themes work/how I can edit/change it? Thanks.

Also can you tell me what the php code for putting total post/comment counts into header.php/a text widget? If you can, can you also please include a/the site where it explains how it works? Thanks.