PHP Manual PHP The Right Way Vagrant
php-o … some function wrappers
Guidelines for theme development: some of this applies to plugins as well.
Messing around with the new media manager
Modern HTML/CSS tutorial
PDF metadata http://framework.zend.com/manual/1.12/en/zend.pdf.info.html
From the CODEX
http://codex.wordpress.org/Writing_a_Plugin
Hints and tips
http://wp.smashingmagazine.com/2011/03/08/ten-things-every-wordpress-plugin-developer-should-know/
Best practices
http://wp.smashingmagazine.com/2010/07/30/lessons-learned-from-maintaining-a-wordpress-plug-in/
Intro catalog
http://wpmu.org/how-to-write-a-wordpress-plugin-12-essential-guides-and-resources/
Coding pitfalls
http://planetozh.com/blog/2009/09/top-10-most-common-coding-mistakes-in-wordpress-plugins/
WordPress coding standards (from the codex)
http://codex.wordpress.org/WordPress_Coding_Standards (single quotes where possible, tab indents, K&R braces,
Setup stuff
Installing server with PHP http://vertstudios.com/blog/php-apache-installation-tutorial/
Setting up to debug http://robsnotebook.com/php_debugger_pdt_xdebug