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

bitly_integration_with_wordpress_1.php

<?php

  add_action('publish_post', function ($post_id) {

    global $wpdb;

    $api_key = 'GENERATED_ACCESS_TOKEN';
    $api_url = 'https://api-ssl.bitly.com/v4/bitlinks';

    $data = array(
      'long_url' => get_permalink($post_id)
    );

    $payload = json_encode($data);

    $header = array(
      'Authorization: Bearer ' . $api_key,
      'Content-Type: application/json',
      'Content-Length: ' . strlen($payload)
    );

    $curl = curl_init($api_url);
    curl_setopt($curl, CURLOPT_CUSTOMREQUEST, 'POST');
    curl_setopt($curl, CURLOPT_POSTFIELDS, $payload);
    curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($curl, CURLOPT_HTTPHEADER, $header);
    curl_setopt($curl, CURLOPT_CONNECTTIMEOUT, 5);

    $result     = curl_exec($curl);
    $resultJSON = json_decode($result);

    if (isset($resultJSON->link)) {
      update_post_meta($post_id, 'bitly_url', $resultJSON->link);
    }
  }, 10, 1);