How can I get the most popular phrases from a lot of text?
<p>I'm setting up a Twitter-style "trending topics" box for my forum. I've got the most popular /words/, but can't even begin to think how I will get popular phrases, like Twitter does.</p>

<p>As it stands I just get all the content of the last 200 posts into a string and split them into words, then sort by which words are used the most. How can I turn this from most popular words into the most popular phrases?</p>
