Continuing in his introductory series of posts Brandon Savage has added this new tutorial to his blog introducing and showing how to work with functions.
For new developers, especially developers who have never been programmers before, moving from writing simple scripts to writing functions is a process that takes time. I developed in PHP for years before I wrote a single function. I also never found a comprehensive tutorial on how functions work, or how to write them. There's documentation in the manual, but it's a bit hard to grasp if you're new.
He looks at what they are, how they're used and some of the more technical details like scoping, optional arguments, using globals and passing values by reference.