There hasn't been a post here for a little while because I have been ill. I am back now though :)

While writing more Bloworm, I needed a function that would automatically detect the url of the favicon that is associated with a given url. I wrote a quick function to do this a while ago - and have been improving it little by little.

I now have it at a point where it finds the correct url 99% of the time, so I thought that I would share it with you.

/*
 * @summary Given a url, this function will attempt to find it's correspending favicon.
 *
 * @returns The url of the corresponding favicon.
 */
function auto_find_favicon_url($url)
{
    if(!validate_url($url))
        senderror(new api_error(400, 520, "The url you specified for the favicon was invalid."));

    // todo protect against downloading large files
    // todo send HEAD request instead of GET request
    try {
        $headers = get_headers($url, true);
    } catch (Exception $e) {
        senderror(new api_error(502, 710, "Failed to fetch the headers from url: $url"));
    }
    $headers = array_change_key_case($headers);

    $urlparts = [];
    preg_match("/^([a-z]+)\:(?:\/\/)?([^\/?#]+)(.*)/i", $url, $urlparts);

    $content_type = $headers["content-type"];
    if(!is_string($content_type)) // account for arrays of content types
        $content_type = $content_type[0];

    $faviconurl = "images/favicon-default.png";
    if(strpos($content_type, "text/html") !== false)
    {
        try {
            $html = file_get_contents($url);
        } catch (Exception $e) {
            senderror(new api_error(502, 711, "Failed to fetch url: $url"));
        }
        $matches = [];
        if(preg_match("/rel=\"shortcut(?: icon)?\" (?:href=[\'\"]([^\'\"]+)[\'\"])/i", $html, $matches) === 1)
        {
            $faviconurl = $matches[1];
            // make sure that the favicon url is absolute
            if(preg_match("/^[a-z]+\:(?:\/\/)?/i", $faviconurl) === 0)
            {
                // the url is not absolute, make it absolute
                $basepath = dirname($urlparts[3]);

                // the path should not include the basepath if the favicon url begins with a slash
                if(substr($faviconurl, 0, 1) === "/")
                {
                    $faviconurl = "$urlparts[1]://$urlparts[2]$faviconurl";
                }
                else
                {
                    $faviconurl = "$urlparts[1]://$urlparts[2]$basepath/$faviconurl";
                }
            }
        }
    }

    if($faviconurl == "images/favicon-default.png")
    {
        // we have not found the url of the favicon yet, parse the url
        // todo guard against invalid urls

        $faviconurl = "$urlparts[1]://$urlparts[2]/favicon.ico";
        $faviconurl = follow_redirects($faviconurl);
        $favheaders = get_headers($faviconurl, true);
        $favheaders = array_change_key_case($favheaders);

        if(preg_match("/2\d{3}/i", $favheaders[0]) === 0)
            return $faviconurl;
    }

    return $faviconurl;
}

This code is pulled directly from the Bloworm source code - so you will need to edit it slightly to suit your needs. It is not perfect, and will probably will be updated from time to time.