How to get meta keywords and description of a web page?

get_meta_tags() function return all meta tags of web page.
For example:
$url = "http://www.example.com";   
$tags = get_meta_tags($url);
$description     = $tags["description"];
$keywords         = $tags["keywords"];
echo 'Keywords: '.$keywords;
echo '<br/>';
echo 'Description: '.$description;