Use Twitter API to export data (in CSV format) on people that are following you. See Installation notes for steps. Prerequisites: 1) Twitter OAuth PHP library 2) Create your own Twitter application and OAuth keys/tokens.

followers.php

<?php

/* followers.php - use Twitter API to export data (CSV format) on people that are following you

    Installation:
    1. Install Twitter OAuth PHP library (https://github.com/abraham/twitteroauth)
    2. Adjust twitteroauth.php include path below
    3. Create Twitter application (https://dev.twitter.com/apps)
    4. Fill in 4 Twitter app keys below
    5. Adjust $fields array if you want different fields saved

    Usage:
    php followers.php > followers.csv

    Modified by:
    Jim Bohne

    Based on friends.php by:
    Brian Cantoni
    <brian AT cantoni DOT org>
    http://www.cantoni.org
*/

include ('./code/twitteroauth/twitteroauth.php');

/* global settings */
ini_set ('display_errors', 1);
error_reporting (E_ALL & ~E_NOTICE);
date_default_timezone_set ('America/New_York');

/* Twitter user fields to write out as CSV */
$fields = array ("name","screen_name","statuses_count","favourites_count","followers_count","friends_count","location","url");

/* Twitter app keys */
define ('CONSUMER_KEY', 'your-value-here');
define ('CONSUMER_SECRET', 'your-value-here');
define ('OAUTH_TOKEN', 'your-value-here');
define ('OAUTH_TOKEN_SECRET', 'your-value-here');

/* create Twitter object, override to use API v1.1 */
$twitter = new TwitterOAuth(CONSUMER_KEY, CONSUMER_SECRET, OAUTH_TOKEN, OAUTH_TOKEN_SECRET);
if (!is_object($twitter)) {
    fwrite (STDERR, "Error creating TwitterOAuth object\n");
    exit (-1);
}
$twitter->host = "https://api.twitter.com/1.1/";

/* main loop: fetch follower ids, then details, then write out as CSV */
fputcsv (STDOUT, $fields);
$cursor = -1; // first page
$follower_total = 0;
while ($cursor != 0) {
    $params = array(
        'stringify_ids' => true,
        'count' => 100,
        'cursor' => $cursor,
    );

    /* pull follower ID numbers, 100 at a time
       docs: https://dev.twitter.com/docs/api/1.1/get/followers/ids
     */
    $followers = $twitter->get("followers/ids", $params);
    if (!is_object($followers) || isset($followers->errors)) {
        fwrite (STDERR, "Error retrieving followers\n");
        print_r($followers);
        exit (-1);
    }

    $ids = implode (',', $followers->ids);
    $cursor = $followers->next_cursor_str;
    $follower_total += count($followers->ids);
    fprintf (STDERR, "Found %d followers in this batch; cursor=%s\n", count($followers->ids), $cursor);

    /* pull follower details, 100 at a time, using POST
       docs: https://dev.twitter.com/docs/api/1.1/get/users/lookup
     */
    $params = array(
        'user_id' => $ids,
    );
    $users = $twitter->post("users/lookup", $params);
    if (!is_array($users)) {
        fwrite (STDERR, "Error retrieving users\n");
        print_r ($users);
        exit (-1);
    }

    foreach ($users as $u) {
        $csv = array();
        foreach ($fields as $f) {
            $csv[] = $u->{$f};
        }
        fputcsv (STDOUT, $csv);
    }

}

fprintf (STDERR, "Done! Found %d followers\n", $follower_total);

Fatal error: Class 'PHPUnit_Framework_TestCase' not found in /home/username/public_html/tests/TwitterOAuthTest.php on line 10

Having this error during runtime.