How to find out who don't follow back with Twitter's REST Api.

<?php
require_once 'env.php';
require "vendor/autoload.php";

use Abraham\TwitterOAuth\TwitterOAuth;
use Tightenco\Collect\Support\Collection;

$api       = new TwitterOAuth(CONSUMER_KEY, CONSUMER_SECRET, ACCESS_TOKEN, ACCESS_TOKEN_SECRET);
$parameter = [
    'screen_name' => 'mnlwldr',
    'count'       => 5000,
];

/* Get a list of IDs who follow me */
$follower   = $api->get("followers/ids", $parameter);

/* Get a list of IDs I follow */
$followings = $api->get("friends/ids", $parameter);

collect($followings->ids)

    /* diff the two arrays and return the difference */
    ->diff($follower->ids)

    /* chunk the diff because Twitter users/lookup accept max 100 IDs per request */
    ->chunk(100)

    /* now iterate over the chunks and call the users/lookup endpoint */
    ->flatMap(function (Collection $ids) use ($api) {
        return $api->get('users/lookup', [
            'user_id' => $ids->implode(','),
        ]);
    })
    /* iterate over the result and print the username */
    ->each(function ($profile) {
        print $profile->screen_name . PHP_EOL;
    });