This is small tutorial showing you how to integrate new URL shortner to Tweet Old Post wordpress plugin
Make the following changes in top-admin.php file
1 – Look for the URL shortner drop down list, find the code that looks like below
2 – Once you get there, add another option tag to the list like below,
<option value="urlshortner">'.__('urlshortner', 'TweetOldPost').'</option>
Note – In the above option replace ‘urlshortner’ with new shortner name.
Make the below changes in top-core.php file,
1 – find the function shorten_url(), and add an elseif block of new shortner api like below code,
elseif ($shortener=="urlshortner") {
$url = "https://u.nu/unu-api-simple?url={$the_url}";
$response = send_request($url, 'GET');
}
Note – replace urlshortner with new shortner name, it should be same as in point 2 of top-admin.php changes. In the URL section change the API URL with your shortner API URL and pass the required parameters the above just have the URL parameter.
2 -Some API may require additional parameters like API key. So it will look somewhat like this,
elseif ($shortener=="urlshortner") {
$url = "https://yourshortnerapiurl/api?url={$the_url}&api=xxxxxx";
$response = send_request($url, 'GET');
}
Note – replace urlshortner with new shortner name, it should be same as in point 2 of top-admin.php changes. In the URL section change the API URL with your shortner API URL and pass the required parameters. The above have the URL parameter and API key replace xxxxxx with your API Key value.
That’s it and you are ready to go. In case you find it difficult to implement the above code feel free to drop by a comment I’ll add it for everyone 
and you can always drop me a mail.