WebページからフィードのURLを抽出する。（DOMDocument版）

getfeedurl.php

<?php

function getFeedUrl( $url ){
	
	$doc = DOMDocument::loadHTMLFile( $url );
	if( empty( $doc ) ){ return false; }

	$types = array(
		"application/x.atom+xml",
		"application/atom+xml",
		"application/xml",
		"text/xml",
		"application/rss+xml",
		"application/rdf+xml"
	);

	$feedUrl = null;
	$links = $doc->getElementsByTagName("link");
	for( $i = 0; $i < $links->length; $i ++ ){
		$link = $links->item( $i );
		if( !stristr( $link->getAttribute("rel"), "alternate" ) ||
			!in_array( $link->getAttribute("type"), $types ) ){ continue; }
		$feedUrl = $link->getAttribute("href");
		if( !empty($feedUrl) ){ break; }
	}
	return $feedUrl;
}