How to fetch rss feed url of a website using php?

Question

I need to find the rss feed url of a website programmatically.

[Either using php or jquery]

Answer 1

The general process has already been answered (Quentin, DOOManiac), so some code (Demo):

<?php

$location = 'http://hakre.wordpress.com/';
$html = file_get_contents($location);
echo getRSSLocation($html, $location); # http://hakre.wordpress.com/feed/

/**
 * @link http://keithdevens.com/weblog/archive/2002/Jun/03/RSSAuto-DiscoveryPHP
 */
function getRSSLocation($html, $location){
    if(!$html or !$location){
        return false;
    }else{
        #search through the HTML, save all <link> tags
        # and store each link's attributes in an associative array
        preg_match_all('/<link\s+(.*?)\s*\/?>/si', $html, $matches);
        $links = $matches[1];
        $final_links = array();
        $link_count = count($links);
        for($n=0; $n<$link_count; $n++){
            $attributes = preg_split('/\s+/s', $links[$n]);
            foreach($attributes as $attribute){
                $att = preg_split('/\s*=\s*/s', $attribute, 2);
                if(isset($att[1])){
                    $att[1] = preg_replace('/([\'"]?)(.*)\1/', '$2', $att[1]);
                    $final_link[strtolower($att[0])] = $att[1];
                }
            }
            $final_links[$n] = $final_link;
        }
        #now figure out which one points to the RSS file
        for($n=0; $n<$link_count; $n++){
            if(strtolower($final_links[$n]['rel']) == 'alternate'){
                if(strtolower($final_links[$n]['type']) == 'application/rss+xml'){
                    $href = $final_links[$n]['href'];
                }
                if(!$href and strtolower($final_links[$n]['type']) == 'text/xml'){
                    #kludge to make the first version of this still work
                    $href = $final_links[$n]['href'];
                }
                if($href){
                    if(strstr($href, "http://") !== false){ #if it's absolute
                        $full_url = $href;
                    }else{ #otherwise, 'absolutize' it
                        $url_parts = parse_url($location);
                        #only made it work for http:// links. Any problem with this?
                        $full_url = "http://$url_parts[host]";
                        if(isset($url_parts['port'])){
                            $full_url .= ":$url_parts[port]";
                        }
                        if($href{0} != '/'){ #it's a relative link on the domain
                            $full_url .= dirname($url_parts['path']);
                            if(substr($full_url, -1) != '/'){
                                #if the last character isn't a '/', add it
                                $full_url .= '/';
                            }
                        }
                        $full_url .= $href;
                    }
                    return $full_url;
                }
            }
        }
        return false;
    }
}

See: RSS auto-discovery with PHP (archived copy).