How to rename automatically image file names while uploading?

Here's my upload function code:

function upload(&$file, $destinationDir = "", $destinationName = "", $secure = true){
    $ret = false;
    if (isset($file['tmp_name']) && isset($file['name'])){
        if ($destinationName == ''){
            $destinationName = $file['name'];
        }

        $destinationFile = $destinationDir . '/' . $destinationName;
        if (move_uploaded_file($file['tmp_name'], $destinationFile)){
            if ($secure){
                chmod($destinationFile, 0644);
            }

            $ret = true;
        }
    }

    return $ret;
}

This function can't rename file names automatically, if two files name are same, it will replace new image file to existing file.

How can I change this above function to rename files automatically while uploading to server?

Thank you very much