class.upload.php is a powerful and mature PHP class to manage uploaded files, and manipulate images in many ways. The script is available under a GPL license.
more info about the class
$handle->file_new_name_body = 'new name';
$handle->file_new_name_body = $_POST['my_field'];
So for instance, you can do:
But be careful to escape the field first, otherwise it is a big security risk!