PHP Upload and Rename File

I have a html upload form here, What is does is displays a browse button so users can upload files to my site, what I want them to be able to do is completely rename the file that they are uploading once they upload it to my site

Html file upload

<form action="" enctype="multipart/form-data" method="post">
 <input id="file" name="file" type="file" />
 <input id="Submit" name="submit" type="submit" value="Submit" />
 </form>

PHP File

<?php

// Upload and Rename File

if (isset($_POST['submit']))
{
 $filename = $_FILES["file"]["name"];
 $file_basename = substr($filename, 0, strripos($filename, '.')); // get file extention
 $file_ext = substr($filename, strripos($filename, '.')); // get file name
 $filesize = $_FILES["file"]["size"];
 $allowed_file_types = array('.doc','.docx','.rtf','.pdf');

if (in_array($file_ext,$allowed_file_types) && ($filesize < 200000))
 { 
 // Rename file
 $newfilename = md5($file_basename) . $file_ext;
 if (file_exists("upload/" . $newfilename))
 {
 // file already exists error
 echo "You have already uploaded this file.";
 }
 else
 { 
 move_uploaded_file($_FILES["file"]["tmp_name"], "upload/" . $newfilename);
 echo "File uploaded successfully."; 
 }
 }
 elseif (empty($file_basename))
 { 
 // file selection error
 echo "Please select a file to upload.";
 } 
 elseif ($filesize > 200000)
 { 
 // file size error
 echo "The file you are trying to upload is too large.";
 }
 else
 {
 // file type error
 echo "Only these file typs are allowed for upload: " . implode(', ',$allowed_file_types);
 unlink($_FILES["file"]["tmp_name"]);
 }
}

?>