Headlines and headings are usually very relevant and descriptive pieces of information for any HTML page. You might want to include them into the description
<meta> tag on that page. Here is a simple regular expression to extract all those headings:
preg_match_all( '|<h[^>]+>(.*)</h[^>]+>|iU', $html, $headings );
$html is the HTML source and
$headings will be an array populated with the extracted headings.