Thankfully, PHP code is some of the easiest to write (of the web scripting languages I've worked with). Unfortunately, documentating it is just as much of a pain as it is with any other language. And, this Builder.com article puts it well: Developers and documentation are like oil and water - they just don't mix well. Many developers find writing documentation to be a boring waste of time. It's no wonder, then, that most companies hire separate personnel to create technical documentation rather than try to persuade developers and programmers to do it themselves.

Thankfully, this article also offers an alternative - phpDocumentor - a tool that creates API documentation based off of a set of special tags/comments already built into the code. With various values allowed besides just the comments (author, version, todo, etc), there's quite a bit of power here. phpDocumentor is really a quite useful tool - I've used it before in freelance projects to create documentation for the end user, and it worked like a charm.