This page is extremely simple: a very basic HTML form. It accepts id parameter via GET.

if (empty($_GET["id"])) {
    die("No author ID provided");
} else {
    $author = Registry::persistenceDriver()->find((int) $_GET["id"], new Author());
    if (!$author) {
        die("Author ID #" . (int) $_GET["id"] . " not found");
    }
}

Form submission is handled by author_save.php:

$author = new Author($_POST);
$author->id = ((int) $author->id) ?: null;
$author->name = trim((string) $author->name);

if ($author->name) {
    Registry::persistenceDriver()->save($author);
    header("Location: authors.php");
    exit;
} else {
    echo "Empty name";
}