使用php上传后如何重命名文件名？

$changeTXT = $_SESSION['username'];
$uploaderName = strtolower($changeTXT);
$changeTXT = strtolower($changeTXT);
$changeTXT = ucfirst($changeTXT);
$filelocation = $_POST['userfile'];
$filename = $_POST['filename'];
$max_size = $_POST['MAX_FILE_SIZE'];

$file = $_FILES['userfile'];

$allowedExtensions = array("wma","mp3","wav");

function isAllowedExtension($fileName) {
  global $allowedExtensions;

  return in_array(end(explode(".",$fileName)),$allowedExtensions);
}

if($file['error'] == UPLOAD_ERR_OK) {
  if(isAllowedExtension($file['name'])) {

$uploaddir = "uploads/".$uploaderName."/";

$uploadfile = $uploaddir . basename($_FILES['userfile']['name']);

if (move_uploaded_file($_FILES['userfile']['tmp_name'],$uploadfile)) {

    echo "Thank you for uploading your music!<br /><br />";

} else {

    echo "Your file did not upload.<br /><br />";

}

    echo "n";

    echo "<a href='index.php'>Return</a> to index.<br /><br />$uploaddir";

} else {

    echo "You have tried to upload an invalid file type.<br /><br />";

  }

} else die("Cannot upload");