I am trying to use apache-rewrite rule to convert the below URL:
http://localhost/foo/bar/news.php?id=24
Into this
Unfortunately I'm unable to answer your question in PHP or Apache (although I am using a hand-rolled REST converter to create URL addresses on my current project), but from what I understand, you want your user to type out http://localhost/DIRECTORY/AID/article.php?a_id=24 and the address bar should end up like http://localhost/DIRECTORY/AID/news/this_is_article_title.
I'm not entirely sure what benefits this provides for you, and please bear in mind my solution will NOT allow your end-user to type the RESTful URL and end up at the page with the QueryString address. You'd need some extra legwork to do this, and even more legwork to keep it in sync with the DB (I'd recommend a script that deseminates the RESTFUL URL, queries the DB for the topic then returns the ID or page content ... but that's an different Stack Overflow question for another day.
SOLUTION My proposed solution requires HTML5 doctype and a very light sprinkling of Javascript.
history.replaceState(null, "history title here", "news/this_is_article_title");
What this does is it changes the URL in the address bar without triggering a page redirect, reload or anything else. This javascript can be dynamically written with your PHP so as the page is served, the address is updated. The higher up in the document it is, the faster the change.
Here's a jsFiddle link: http://jsfiddle.net/uT3RP/1/
Unfortunately they run the code in an iframe so it doesn't control the main address bar, so I included an alert displaying what the address bar location would say, for proof. It's not the most elegant of solutions. I wouldn't even recommend doing what you're doing without the failsafes of making sure the URL displayed can be used to get to the same page. Disclaimer aside ... your problem is solved with 1 line of js and a doctype change (if you aren't using HTML5).
You can stop flogging poor ol' htaccess and get on with your project :)