preg_match('/\<div id\=[\"]{0,1}ad_content[\"]{0,1}\>(.*?)\<\/div\>/s', $content['content_body'], $matches);
$content['content_body'] = $matches[1];

This allows you to get contents of any div with ID tag. 
$matches[0] returns with parent div tag
$matches[1] returns with out parent div tag

Regular Expression to get contents of div[id]

regex, php

PHP