It's annoying and surprising to have to type @param string|null instead of @param ?string.

Checking if the first character of a type is ? and stripping that (and adding null) may be the quickest workaround, not sure if upgrading dependencies would help.

https://github.com/phpDocumentor/TypeResolver/ issue 31 seems related, phpdocumentor2 (project) does parse these.