jeje, aquí está el código php que utiliza la parte de que contiene el formulario html para subir los archivos.
 
<?php
if ($_POST) {
    require 'DropboxUploader.php';  
    try {
        // Rename uploaded file to reflect original name
        if ($_FILES['file']['error'] !== UPLOAD_ERR_OK)
            throw new Exception('File was not successfully uploaded from your computer.'); 
        $tmpDir = uniqid('/tmp/DropboxUploader-');
        if (!mkdir($tmpDir))     //AQUI ME MARCA EL ERROR. line 21
            throw new Exception('Cannot create temporary directory!'); 
        if ($_FILES['file']['name'] === "")
            throw new Exception('File name not supplied by the browser.'); 
        $tmpFile = $tmpDir.'/'.str_replace("/\0", '_', $_FILES['file']['name']);
        if (!move_uploaded_file($_FILES['file']['tmp_name'], $tmpFile))
            throw new Exception('Cannot rename uploaded file!'); 
        // Enter your Dropbox account credentials here
		$uploader = new DropboxUploader('
[email protected]', 'gekko.21');
        $uploader->upload($tmpFile, $_POST['dest']); 
        echo '<span style="color: green;font-weight:bold;margin-left:393px;">File successfully uploaded to my Dropbox!</span>';
    } catch(Exception $e) {
        echo '<span style="color: red;font-weight:bold;margin-left:393px;">Error: ' . htmlspecialchars($e->getMessage()) . '</span>';
    } 
    // Clean up
    if (isset($tmpFile) && file_exists($tmpFile))
        unlink($tmpFile); 
    if (isset($tmpDir) && file_exists($tmpDir))
        rmdir($tmpDir);
}
?>  
No sé si se puedan sjubir los archivos para que los vean.