Move $upload_dir
at the top, as you are calling it before you initialize it.
$upload_dir = "images/external/doodles/";
$url = md5(uniqid(rand(), true));
$unique_user_id = $_SESSION['unique_user_id'];
$unique_post_id = md5(uniqid(rand(), true));
$timestamp = time();
$nature = "doodle";
$imageUrl = $upload_dir.$url.'.png';