Complete inline documentation comment blocks (docblocks) must be provided. Please read the Sample File and Header Comment Blocks sections of the PEAR Coding Standards to learn the specifics of writing docblocks. Further information can be found on the phpDocumentor website.
Non-documentation comments are strongly encouraged. A general rule of thumb is that if you look at a section of code and think "Wow, I don't want to try and describe that", you need to comment it before you forget how it works.
C style comments (/* */) and standard C++ comments (//) are both fine. Use of Perl/shell style comments (#) is discouraged.