favicon-links.php

<?php

/**
 * Plugin Name: Favicon Links
 * Plugin URI: http://whyte624.ru/prj/wordpress-favicon-links
 * Description: Adds favicons to all links in any post
 * Version: 1.1
 * Author: Andrei Sozonov
 * Author URI: http://whyte624.ru/
 * License: GPL2
 */

/**
 * @param $content
 * @return string
 */
function favicon_links_the_content($content)
{
    try {
        $html = new DOMDocument(null, 'UTF-8');
        @$html->loadHTML('<meta http-equiv="content-type" content="text/html; charset=utf-8">' . $content);

        foreach ($html->getElementsByTagName('a') as $a) {
            /* @var DOMElement $a */
            $imgs = $a->getElementsByTagName('img');
            if ($imgs->length > 0) {
                continue;
            }

            $url = $a->attributes->getNamedItem('href');
            if (!$url) {
                continue;
            }
            $urlParts = parse_url($url->textContent);
            if (!$urlParts || empty($urlParts['host'])) {
                continue;
            }

            if ($a->attributes->getNamedItem('data-no-favicon')) {
                continue;
            }

            $img = $html->createElement('img');
            $img->setAttribute('src', 'http://www.google.com/s2/favicons?domain=' . $urlParts['host']);
            $img->setAttribute('style', 'display: inline-block; padding-right: 4px;');
            $a->insertBefore($img, $a->firstChild);
        }

        return $html->saveHTML();
    } catch (Exception $e) {
        return $content;
    }
}

add_filter('the_content', 'favicon_links_the_content');