Adherence to PEAR CS is good, and overall I like what you've done; but there are a few things you need to do before you entirely get my vote.
* Docblocks need to be updated - the constructor's "@return" should be "@return PHP_Mailman", for example.
* This package doesn't fit into the PHP category - this is a Service package that integrates with Mailman, not one that does PHP specific 'work'.
* Use http_build_query rather than hardcoding query strings.
* Don't parse HTML using regular expressions - this is a very brittle approach