An rdoc is only as useful as it's included use-cases. About the best thing about PHP is the fact that there are zillions of included comments on the php documentation pages. While most are not at all what should run in production, a single good comment can give you a lot of insight as to how to use some code.
Tests != actual usage.
This is actually why a readme for a gem, plugin, extension, module etc. is so important, and why I think Github has been good for the developer community at large.
This has been my experience developing with PHP for many years. Especially in the early years, I found more code in the comments on PHP documentation than anywhere else (in documentation, on blogs, on Q&A sites). That the PHP community has always had this ability for its millions of users to aggregate code for common issues in one place makes its documentation much more extensive than it may appear at first glance.