Use Bitly to generate short URLs in WordPress. Visit blog post https://bit.ly/2XB3M8s

bitly_integration_with_wordpress_2.php

<?php

  remove_action('wp_head', 'wp_shortlink_wp_head', 10, 0);

  add_action('wp_head', function() {

    global $post;

    $short_url = get_post_meta($post->ID, 'bitly_url', true);

    if(!empty($short_url)) {
      echo '<link rel="shortlink" href="' . $short_url . '" />';
    } else {
      echo '<link rel="shortlink" href="' . get_bloginfo('url') . '?p=' . $post->ID . '" />';
    }
  }, 10, 1);

  add_action('pre_get_shortlink', function() {

    global $post;

    $short_url = get_post_meta($post->ID, 'bitly_url', true);

    if(!empty($short_url)) {
      return $short_url;
    } else {
      return get_bloginfo('url') . '?p=' . $post->ID;
    }
  }, 10, 1);