I wrote a script for that, but it is not very general. It works for us because we did not use MW templates and other advanced constructs. It might give you some ideas at least:

https://github.com/Symplicity/phabricator/blob/master/scripts/util/import_mediawiki.php

The config file is something like this:

{
  "wiki.url" : "https://wiki.foo.com",
  "category.map" : {
    "Foo" : "projects/foo",
    "JavaScript" : "javascript",
    ...
  },
  "conduit.user" : "mwbot",
  "conduit.cert" : "..."
}

See T3179.