You want to actually automate the problem of filling in the "javadoc" type data?
The DMS Software Reengineering Toolkit could be configured to do this.
It parses source text just like compilers do, builds internal compiler structures, lets you implement arbitrary analyses, make modification to those structures, and then regenerate ("prettyprint") the source text changed according to the structure changes. It even preserves comments and formatting of the original text; you can of course insert additional comments and they will appear and this seems to be your primary goal. DMS does this for many languages, including PHP
What you would want to do is parse each PHP file, locate every class/method, generate the "javadoc" comments that should be that entity (difference for classes and methods, right?) and then check that corresponding comments were actually present in the compiler structures. If not, simply insert them. PrettyPrint the final result.
Because it has access to the compiler structures that represent the code, it shouldn't be difficult to generate parameter and return info, as you suggested. What it can't do, of course, is generate comments about intendend purpose; but it could generate a placeholder for you to fill in later.
