phpDocumentor is the world standard auto-documentation tool for PHP. Written in PHP, phpDocumentor can be used directly from the command-line. phpDocumentor can be used to generate professional documentation directly from the source code of your PHP project. Support for linking between documentation, automatic class inheritance and generation of highlighted source code with cross-referencing to php general documentation are just a few of the features of phpDocumentor.
In any kind of software development there will be code reuse, whether intended or not. Source code documentation, the comments placed in your php files, can act as an explanation for behavior. Without some kind of documentation, users are forced to make assumptions about code, or find things out using trial and error, or worst of all, fill your mailbox with requests. But even if you've documented your code well picking comments out of lines and lines of code can be tedius, and not very useful.