Newbie Question: breaking a substr() string on a space

Hi, everyone, I'm 'teasers' of the first, say 200 to 250 characters of some articles. This works great:

    $content = strip_tags($article['content']);
    $striptease = substr($content, 0, 275);

but cuts off words right in the midd ... I'd like to find a way to take $content, look at the first 250 characters, then count backwards to the last space and chop it there. Does anyone have an ideas/scripts for this? Thanks in advance, Jack