I've been having a look into this. I think a few changes will need to be made to make this happen.

1. I think a new `returntype` needs to be added in the formatter classes in this package. eg `Package_Generic_XHTML`. I don't think `type` can be reused as it is used anywhere a type is mentioned in the docs, not just return types, and the return type needs to be formatted differently. eg `sprintf(' : <span>%s</span>')`

2. I think the docbook schema in the `phpdoc` repository needs to be updated to allow a new `returntype` element.

```
  <methodsynopsis>
   <methodname>json_decode</methodname>
   <methodparam><type>string</type><parameter>json</parameter></methodparam>
   <methodparam choice="opt"><type>bool</type><parameter>assoc</parameter><initializer>false</initializer></methodparam>
   <methodparam choice="opt"><type>int</type><parameter>depth</parameter><initializer>512</initializer></methodparam>
   <methodparam choice="opt"><type>int</type><parameter>options</parameter><initializer>0</initializer></methodparam>
   <returntype>mixed</returntype>
  </methodsynopsis>
``` 
3. All the existing documentation needs to be updated to the new format. :O

Let me know if I'm on the right tracks. Thanks.

Probably best asking in the docs group:

http://news.php.net/group.php?group=php.doc

https://secure.php.net/mailing-lists.php

Or r11.

We should be able to do this without touching the source XML files, by changing how and where PhD renders the return-type-specific <type> elements. It has been a while since I touched PhD but it feels like something that can be done relatively easily.

We definitely want to avoid adding new elements to DocBook (which isn't something our project owns: see http://docbook.org/) and/or touching XML files en-masse (which makes life more difficult for the documentation translation team).

I've pushed a PR that should hopefully be a good starting point.
See https://github.com/php/phd/pull/17

Merged above PR.