A quick reference guide for PHP, with functions references, a regular expression syntax guide and a reference for PHP's date formating functions. As of 28/6/14, the cheat sheet now includes popup links to the appropriate PHP manual pages.
A handy guide to the basic principles of Object Oriented Programming and Design. Covers standard and accepted principles, design patterns, and more! This is not intended to teach OOP, but as a reminder for those who already learned.