// set start and end tags
$starttag = '<a href="http';
$starttagsub = 'http';
$endtag = '"';

// escape characters that would mess up preg_match_all
$starttagesc = preg_quote($starttag, '/');
$endtagesc = preg_quote($endtag, '/');

// match all instances and assign value to $results array
preg_match_all("/$starttagesc.*?$endtagesc/", $post_body, $results);

// example result
// <a href="http://www.php.net/download-php.php3?csel=br

// loop through array
foreach($results as $key => $value)
	{
	// remove the href code so we're just left with the link
	$sp = strpos($value, $starttag, 0) + strlen($starttag) - strlen($starttagsub);
	$ep = strrpos($value, $endtag);
	$link = substr($value, $sp, $ep-$sp);

	// use PHP's parse_url function
	// eg: http://www.php.net/download-php.php3?csel=br
	// $url['scheme'] = http
	// $url['host'] = www.php.net
	// $url['path'] = /download-php.php3
	// $url['query'] = csel=br
	$linkbits = parse_url($link);
	$host = $linkbits['host'];

	// find 'www.'
	$pos = strpos($host, 'www.');

	// if not found
	if ($pos === false)
		{
		$domains_http[] = $host;
		}
	
	// if found
	else
		{
		$domains_http[] = substr($host, $pos + 4);
		}
	}

// if array
if (is_array($domains_http))
	{
	// remove duplicates
	$domains_http = array_unique($domains_http);
	}