Write-only file uploads for PHP server

pobox.php

<?php
define("UPLOAD_DIR", "/tmp/pobox-uploads/");
define("CODE", "secretcodehere");

if ($_SERVER['REQUEST_METHOD'] == "POST") {
    if ($_POST["code"] !== CODE) {
        die("upload denied: illegal code");
    }

    if (!(is_dir(UPLOAD_DIR) || mkdir(UPLOAD_DIR, 0700))) {
        die("couldn't create upload dir " . UPLOAD_DIR);
    }

    $file_name = $_FILES['file']['name'];
    $file_size = $_FILES['file']['size'];
    $file_temp = $_FILES['file']['tmp_name'];
    $file_error = $_FILES['file']['error'];

    $new_name = 'pobox-upload-' . date('Y-m-d_H.i.s') . '_' . $file_name . '.upload';

    if ($file_size <= 20 * 1024 * 1024) {
        if(move_uploaded_file($file_temp, UPLOAD_DIR . $new_name)) {
            echo 'File uploaded: '.$file_name;
        } else {
            die('Error: '.$file_error);
        }
    } else {
        die("This file is too big!");
    }

} else {
?>

<form method=post enctype="multipart/form-data">
<input type=file name=file>
<p>Code: <input name=code>
<p> <input type=submit>
</form>

<?php

}


?>