redirect.php

<?php

require 'bootstrap.php';
use Abraham\TwitterOAuth\TwitterOAuth;

/* Build TwitterOAuth object with client credentials. */
$connection = new TwitterOAuth(CONSUMER_KEY, CONSUMER_SECRET);

/* Get temporary credentials. */
$request_token = $connection->oauth('oauth/request_token', ['oauth_callback' => OAUTH_CALLBACK]);

/* If last connection failed don't display authorization link. */
switch ($connection->getLastHttpCode()) {
    case 200:
        /* Save temporary credentials to session. */
        $_SESSION['oauth_token'] = $request_token['oauth_token'];
        $_SESSION['oauth_token_secret'] = $request_token['oauth_token_secret'];

        /* Build authorize URL and redirect user to Twitter. */
        $url = $connection->url('oauth/authorize', ['oauth_token' => $request_token['oauth_token']]);
        break;
    default:
        /* Show notification if something went wrong. */
        echo 'Could not connect to Twitter. Refresh the page or try again later.';
}

// Redirect to authorize URL
header('Location: '.$url);
exit;