Upload and rename file in php?

[Roma]

I want to know about how to rename the file that I have uploaded.
For example I have uploaded test.doc, and i need the file name changed to abc.doc

I have index.php:

<form enctype="multipart/form-data" action="upload.php" method="post">
<pre lang="html">
Upload Modul Low : <form enctype="multipart/form-data" action="upload.php" method="post">
<input name="userfile" type="file" />
<input type="submit" value="Upload" />
</form>

and I have upload.php:
<?php

$limit_size=330000;
$uploaddir = 'upload/';


$uploadfile = $uploaddir . $_FILES['userfile']['name'];
$uploadfile= strtolower(str_replace(" ","_",$uploadfile));



if($uploadfile !=none)
{

$file_size=$_FILES['userfile']['size'];
$file_size_kb= number_format($file_size/1024, 1, ',', '.');
$file_name=$_FILES["userfile"]["name"];
$file_name= strtolower(str_replace(" ","_",$file_name));


if($file_size >= $limit_size){
echo "Error!!<br />The file is too big<br />";
echo "Your file size : ". $file_size_kb ." Kb<br />";
echo "Max Size: 300 Kb";
}

// Proses upload dan informasi hasil upload.
else {
if (move_uploaded_file($_FILES['userfile']['tmp_name'], $uploadfile)) {
echo "File sudah sukses diupload, have fun!<br />";
echo "Nama : " . $file_name . "<br />";
echo "Ukuran : " . $file_size_kb . " Kb<br />";
} else {
echo "Error!!<br />Proses upload gagal, ulangi lagi!" ;
}
}
}


?>