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
$foo->process('/path/to/local/file.jpg');
But apparently, there is a PHP wrapper for it here. In the cade there for PHP, it seems that it actually populates $_FILES.
In any case, with the class, you can process local files:
Wouldn't it be a solution?