Suggestions On Learning WP-Useful PHP?

This might be an unusual question to ask everyone, but what the hey. 🙂

Does anyone have any suggestions on where/how/examples to learn WP-useful PHP from?

For instance, here are some useful functionalities in specific WP themes I have encountered that are, to my knowledge, PHP-specific:

– making consecutive posts or comments odd and even numbered accordingly, which therefore allows for separate CSS stylization of odd and even numbered posts

– breadcrumb trails and showing that you are viewing page 3 out of 10 pages worth of content or something similar

I’m not completely new to PHP and can code a bit on my own and can read (mostly) alter and modify (sometimes, depending) other people’s codes, but I would love to be more proficient at it so I can write code without ‘cheat sheets’.

Problem is… I honestly don’t know where to start to learn such useful things. I’m familiar with JavaScript and I’m an (X)HTML and CSS junkie if that helps any.

Anyone have any pointers or guidance or articles or examples that they’d care to share?

Please and thank you!