In JS, there's a static method OO.ui.Element.getDir that is used to determine direction. It goes like this:

OO.ui.Element.getDir = function ( obj ) {
	<blah blah>
	<'obj' is usually 'document.body' at this point>
	return $( obj ).css( 'direction' );
};

In PHP, we have this so far:

class OoUiElement extends OoUiTag {
	<snip>
	public static function getDir() {
		// TODO: Figure out a way to override this functionality when used within MediaWiki, and use
		// $wgLang->getDir() to get the user interface direction
		return 'ltr';
	}
}

Subclassing OoUiElement doesn't look viable, so that leaves us with the option of adding OoUiElement::setDir() method that the environment may call to set things up (similarly to how one calls OoUiTheme::setSingleton( new OoUiMediaWikiTheme() )).

If we also did this in JS, and used that value as a fallback, and went a bit further in that direction, it could probably become possible to use JS OOUI server-side, too; so that might not be a bad idea in general.

Change 170341 had a related patch set uploaded by Bartosz Dziewoński:
OOUI PHP: Support RTL text direction

https://gerrit.wikimedia.org/r/170341

There's also the bigger question of whether we want to support per-document or per-element directionality.

Currently JS sorta-kinda supports per-element directionality, but it's largely by chance – per comment 1, we just look at CSS 'direction' property and use that. We don't support changing the direction of existing elements. I don't think anybody has tried actually using this and seeing what happens when you mix-and-match.

I discussed it with Moriel on IRC and for now we're only going to support per-document directionality in PHP, and implement per-element support when it turns out that we actually need it.

Change 170341 merged by jenkins-bot:
OOUI PHP: Support RTL text direction

https://gerrit.wikimedia.org/r/170341