Automatic short URL in posts

I installed yourls on a custom short domain. Then added an acf field to posts. Then when a post is added or modified, call yourls API to get the shortlink, and save that link in the custom field

add_action('acf/save_post', 'update_related_posts', 20);

function update_related_posts($post_id) {
    global $old_field_value;

    // Check if the save is an auto-save, in which case we don't want to run this function
    if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) {
        return;
    }

    // Check if the current user has permission to edit the post
    if (!current_user_can('edit_post', $post_id)) {
        return;
    }

    // Check if the saved post is of the desired post type
    if (get_post_type($post_id) !== 'post') {
        return;
    }

    // Get submitted values.
    $new_values = $_POST['acf'];

    // Important add your 'your_field_key' (starting with "field_"), not the 'your_field_name'
    $new_field_value = $new_values['field_64f5a1b107cec'] . $new_values['field_64f5a4153cdf5'] . $new_values['field_6504365a21af0'];

    // Check if the field has changed
    if ($old_field_value != $new_field_value) {

    $api_url="http://mndn.com/yourls-api.php";
    $share_url = get_sample_permalink($post_id);
    $permalink = preg_replace('/\%postname\%/', $share_url[1], $share_url[0]);

    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, $api_url);
    curl_setopt($ch, CURLOPT_HEADER, 0);            // No header in the result
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true); // Return, do not echo result
    curl_setopt($ch, CURLOPT_POST, 1);              // This is a POST request
    curl_setopt($ch, CURLOPT_POSTFIELDS, array(     // Data to POST
            'url' => $permalink,
            'format'   => 'json',
            'action'   => 'shorturl',
            'signature' => '123456789'
        ));
    $data = curl_exec($ch);
    curl_close($ch);

    $data = json_decode( $data );
    //echo $data->shorturl;
    update_field( 'shorturl', $data->shorturl, $post_id);
    }
}

Then append the ACF field value to the_content()

function prefix_add_content ($content){
    if ( !is_singular( 'post' ) || is_admin() ) {
        return;
    }

    $shortlink = '<p><a href="' . get_field( 'shorturl', $post_id ) . '" target="_blank">' . get_field( 'shorturl', $post_id ) . '</p>';
    $content = $content . $shortlink;

    return $content;
}
add_filter ('the_content', 'prefix_add_content');