Count post view counts in WordPress via AJAX (Compatible with caching plugins)

spp.php

<?php
/*
Plugin Name: Simple Popular Posts Lite
Plugin URI: -
Description: -
Version: 2015.03.01
Author: khromov
Author URI: http://snippets.khromov.se
License: GPL2
*/

/**
 * Class Simple_Popular_Posts
 */
class Simple_Popular_Posts
{
    public static $td = 'spp';
    public static $post_types = array('post'); //,'page'

    /**
     * Constructor sets up all our hooks
     */
    function __construct()
    {
        add_action('wp_footer', array(&$this, 'footer_script'), 999);
        add_filter('query_vars', array(&$this, 'query_vars'));
        add_action('wp', array(&$this, 'count'));
    }

    /**
     * Adds counting code to footer
     */
    function footer_script()
    {
        if((is_single() || is_attachment() || is_page() || is_singular()) && in_array(get_post_type(get_the_ID()), $this::$post_types)) :
            ?>
            <script async type="text/javascript">
                function SimplePopularPosts_AddCount(id, endpoint)
                {
                    var xmlhttp;
                    var params = "/?spp_count=1&spp_post_id=" + id + "&cachebuster=" +  Math.floor((Math.random() * 100000));

                    if (window.XMLHttpRequest)
                        xmlhttp = new XMLHttpRequest();
                    else
                        xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");

                    xmlhttp.onreadystatechange = function()
                    {
                        if (xmlhttp.readyState == 4 && xmlhttp.status == 200)
                        {
                            //alert(xmlhttp.responseText);
                        }
                    };

                    xmlhttp.open("GET", endpoint + params, true);
                    xmlhttp.send();
                }
                SimplePopularPosts_AddCount(<?php echo get_the_ID(); ?>, '<?php echo get_site_url(); ?>');
            </script>
        <?php
        endif;
    }

    /**
     * Adds our special query var
     */
    function query_vars($query_vars)
    {
        $query_vars[] = 'spp_count';
        $query_vars[] = 'spp_post_id';
        return $query_vars;
    }

    /**
     * Count function
     *
     * TODO: This should be doable with SHORTINIT and would be much faster
     */
    function count()
    {
        /**
         * Endpoint for counting visits
         */
        if(intval(get_query_var('spp_count')) === 1 && intval(get_query_var('spp_post_id')) !== 0)
        {
            //JSON response
            header('Content-Type: application/json');
            $id = intval(get_query_var('spp_post_id'));

            $current_count = get_post_meta($id, '_spp_count', true);

            if($current_count === '')
                $count = 1;
            else
                $count = intval($current_count)+1;

            //Update post meta
            update_post_meta($id, '_spp_count', $count);

            echo json_encode(array('status' => 'OK', 'visits' => intval($current_count)+1));
        }
    }
}

$simple_popular_posts = new Simple_Popular_Posts();

@murugeasamgr Thanks, I updated the gist. It's really old, it could be rewritten to use fetch as well.