- Please run phpcs on your code
- I'd prefer if you used simplexml+xpath instead of regex (and dom::loadhtml if necessary)
- why don't you throw exceptions? currently I have to check the return value of every single method call manually instead of putting a try-catch around all of it. __construct also does swallow errors that occur in the called methods since it cannot return them.
- I'd remove the default values of your class variables. Put them in the docblock ("'http://www.example.co.uk/mailman/admin';") or in your validation ("passwords-cannot-have-spaces").