Display Popular Posts by Views in WordPress without a Plugin

whpp_track_post_views.php

<?php

function whpp_track_post_views($post_id) {
    if (!is_single())
        return;
    if (empty($post_id)) {
        global $post;
        $post_id = $post->ID;
    }
    whpp_set_post_views($post_id);
}

add_action('wp_head', 'whpp_track_post_views');

function whpp_set_post_views($post_id) {
    $count_key = 'whpp_track_post_views';
    $count = get_post_meta($post_id, $count_key, TRUE);
    if ($count == '') {
        $count = 0;
        delete_post_meta($post_id, $count_key);
        add_post_meta($post_id, $count_key, '0');
    } else {
        $count++;
        update_post_meta($post_id, $count_key, $count);
    }
}

//To keep the count accurate, lets get rid of prefetching
remove_action('wp_head', 'adjacent_posts_rel_link_wp_head', 10, 0);