PHP "people" don't really know how to document things very well. At least, not when compared to Perl people. Generally, I've always found documentation by Perl people to be good, fairly complete, and looks like it's written by a technical person. At the very least, there is usually a good example in it which you can "just use", for the most part. Some PHP documentation I have been having to read (outside the PHP manual) looks like it's written by people who have never actually read documentation. Incomplete, silly errors, and a big PITA to actually read all the side-documentation to figure out what they are talking about.

When you are documenting "How to do X", don't say you need to understand Y, which points you to W. Explain how to do X without needing to know Y and W, and refer people to Y and W to "learn more", rather than a pre-requisite.