Using Tidy from PHP

Apologies for a newbie question ....

I want to use Tidy to strip out Word formating from a little CMS I've coded.
I've Googled for ages now but can't find any tutorials etc :-(

Given that the 'content' (along with the Word formating) is called $content
how do I send that to HTML Tidy and retreive it back to the script ?

TIA

Huw