Until a few years ago, there were basically two tools you could use to generate API documentation in PHP: phpDocumentor and Doxygen. phpDocumentor was long considered the standard, with Doxygen getting notice when more advanced features such as inheritance diagrams are required. However, phpDocumentor is practically unsupported at this time (though a small group of developers is working on a new version), and Doxygen has never had PHP as its primary concern. As such, a number of new projects are starting to emerge as replacements.
One of these is DocBlox. I am well aware there are several others -- and indeed, I've tried several of them. This post is not here to debate the merits or demerits of this or other solutions; the intention is to introduce you to DocBlox so that you can evaluate it yourself.