WordPress integration for VK or other social via url prefix

wp-vkontakte-integration.php

<?php

new WP_VK_Templater();

/**
 * Class WP_VK_Templater
 *
 * Add 'vk/' prefix to al links, and load for such links templates
 * with '-vk' suffix: index-vk.php, page-vk.php, taxonomy-{name}-vk.php, etc.
 */
class WP_VK_Templater
{
    const PREFIX = 'vk';

    function __construct()
    {
        add_filter('rewrite_rules_array', array(&$this, 'rewrite_rules_array_filter'), 100);
        add_filter('query_vars', array(&$this, 'query_vars_filter'));
        add_action('wp_loaded', array(&$this, 'wp_loaded_action'));
        add_action('template_redirect', array(&$this, 'template_redirect_action'));
    }

    function rewrite_rules_array_filter($rules)
    {
        $prefix = static::PREFIX;

        $vk_rules = array();

        $vk_rules["^$prefix/?\$"] = "index.php?is_$prefix=1";

        foreach ($rules AS $rule => $query)
        {
            $vk_rules["$prefix/$rule"] = rtrim($query, '&=') . "&is_$prefix=1";
        }

        $rules = array_merge($vk_rules, $rules);

        return $rules;
    }

    function wp_loaded_action()
    {
        $prefix = static::PREFIX;
        
        $rules = get_option('rewrite_rules');

        if (!isset($rules["^$prefix/?\$"]))
        {
            global $wp_rewrite;
            $wp_rewrite->flush_rules();
        }
    }

    function query_vars_filter($vars)
    {
        $prefix = static::PREFIX;
        array_push($vars, "is_$prefix");

        return $vars;
    }

    function permalink_filter($url)
    {
        $prefix = static::PREFIX;
        $site_url = trailingslashit(get_option('home'));

        $url = preg_replace("~$site_url~iu", $site_url . "$prefix/", $url);

        return $url;
    }

    function template_redirect_action()
    {
        $prefix = static::PREFIX;

        if(!get_query_var("is_$prefix"))
        {
            return;
        }

        add_filter('template_include', function ($template_include)
        {
            $prefix = static::PREFIX;

            //Try load template with VK suffix
            //index-vk.php, page-vk.php, taxonomy-{name}-vk.php, etc.
            $vk_template = trailingslashit(dirname($template_include)) . basename($template_include, '.php') . "-$prefix.php";

            return is_file($vk_template) ? $vk_template : $template_include;
        }, 100, 1);

        add_filter('post_link', array(&$this, 'permalink_filter'), 1, 2);
        add_filter('post_type_link', array(&$this, 'permalink_filter'), 1, 2);
        add_filter('page_link', array(&$this, 'permalink_filter'), 1, 2);
        add_filter('tag_link', array(&$this, 'permalink_filter'), 1, 2);
        add_filter('term_link', array(&$this, 'permalink_filter'), 1, 2);

        //VK  home_url prefix
        add_filter('home_url', function ($url, $path)
        {
            $prefix = static::PREFIX;
            $site_url = trailingslashit(get_option('home'));

            // apply this for home url - not for posts or page permalinks since this filter is called there too
            if (did_action('template_redirect') AND trailingslashit($url) == $site_url)
            {
                $url = $site_url . "$prefix/";
            }

            if ($path == '/')
            {
                $url = $site_url . "$prefix/";
            }

            return $url;
        }, 20, 2);
    }
}

/**
 * Return site base url
 *
 * @param string $path
 * @return string
 */
function site_base_url($path = '')
{
    static $base_url;

    if ($base_url === null)
    {
        $base_url = untrailingslashit(get_option('home'));
    }

    return $path ? $base_url . '/' . ltrim($path, '/') : $base_url;
}