PHP List Authors Alphabetically with First Letter as Header

Easily list all authors of a blog in alphabetical order, grouped by their first name with a single letter as a header character.

function listAuthorsAlphabetically() {
    $users = get_users('orderby=user_login&role=author');
    $firstLetter = '';

    foreach ($users as $user) {
        $space = strpos($user->user_login, ' ');
        $letter = substr($user->user_login, 0, 1);
        $letter = strtoupper($letter);

        if ($letter !== $firstLetter) {
            $firstLetter = $letter;
            echo "<h4 id='ft_contrib_alphaletter_$firstLetter'>$firstLetter</h4>";
        }

        echo '<a href="' . get_author_posts_url($user->ID, $user->user_nicename) . '" title="' . $user->display_name . '">' . $user->display_name . '</a>';
        echo '<br>';
    }
}