The Observer design pattern is very common, and luckily is a very easy concept to understand. Here it is explained!
The front controller is a simple to understand design pattern where you have a main controller that handles every request for a website. It is a commonly used design pattern for many MVC based web applications.
An explanation of the S.O.L.I.D. principles, and how to use them in your PHP scripts.
The decorator pattern explained, with a full and easy to understand example (using PHP).
Adding a background image to a <div> (or any other tag) is easy using CSS. Use these example rules to get a nice gradient that goes over the top of a background image, without adding any additional divs. All in CSS.
A quick into and easy to understand explanation about SQL injection, and what steps you need to do to prevent this affecting your websites.