The ArticleContentArea extension allows users to set the main content area of an article through a parser hook named "articlecontentarea". The extension inserts said content area in a few places:

• the page_props table, which allows querying through the API
• Javascript config variables, as wgArticleContentArea

• Use {{#articlecontentarea:the_actual_article_type}} in an article.
• You can read the property from PHP this way:

    ArticleContentArea::getArticleContentArea( $this->getTitle() );

• You can read the article's content area from JS using mw.config.get('wgArticleContentArea').
• You can read the article's content area using the API this way:

  api.php?action=query&titles=__article_title__&prop=pageprops&ppprop=ArticleContentArea
  

• You can use ArticleContentArea::getJoin( $contentArea ) in other queries to get the content area. This assumes the existence of the page table in the original query. A field aliased 'content_area' is added. $contentArea is null by default, and if passed the join is an inner join, for filtering.

• $wgArticleContentAreaCategoryName - this is the name of a category whose members are valid content areas. By default, it is null, and any content area is valid.

bugfix: the wrong page property was used (copied over from extension:ArticleType)

Internal refactoring and some syntactic sugar in public functions

Initial release