hvis fil eksisterer

Hej

Jeg har dette script men har et problem når jeg uploader et billede så bliver det der allerede ligger på serveren slettet altså hvis de har samme fil navn, er der en måde at komme uden om det og hvis ja vil i så være søde at vise mig hvordan, jeg er mega ny med php. på forhånd tak

<?php

// if an image is uploaded
if (isset ($_POST['submit']))
{
// the thumbnail height
$size = 150;
// the directory where the original uploaded image is saved
$filedir = 'pics/';
// the directory where the thumbnail image is saved
$thumbdir = 'pics/';
// the prefix to be added to the original name to name the thumbnail
$prefix = 'small_';
// the file settings for the uploaded image
$mode = '0666';

$userfile_name = $_FILES['image']['name'];
$userfile_tmp = $_FILES['image']['tmp_name'];
$userfile_size = $_FILES['image']['size'];
$userfile_type = $_FILES['image']['type'];

$userfile_description = isset($_POST['description']) ? $_POST['description'] : "";

if (isset ($userfile_name))
{
$prod_img = $filedir . $userfile_name;
$prod_img_thumb = $thumbdir . $prefix . $userfile_name;
move_uploaded_file($userfile_tmp, $prod_img);
chmod($prod_img, octdec($mode));

$sizes = getimagesize($prod_img);
$aspect_ratio = $sizes[1] / $sizes[0];
if ($sizes[1] <= $size)
{
$new_w = $sizes[0];
$new_h = $sizes[1];
}
else
{
$new_h = $size;
$new_w = abs($new_h / $aspect_ratio);
}
$dest = imagecreatetruecolor($new_w, $new_h) or die('Problem In Creating image');
$src = imagecreatefromjpeg($prod_img) or die('Problem In opening Source Image');
if (function_exists('imagecopyresampled'))
{
imagecopyresampled($dest, $src, 0, 0, 0, 0, $new_w, $new_h, imagesx($src), imagesy($src)) or die('Problem In resizing');
}
else
{
Imagecopyresized($dest, $src, 0, 0, 0, 0, $new_w, $new_h, imagesx($src), imagesy($src)) or die('Problem In resizing');
}
imagejpeg($dest, $prod_img_thumb, 90) or die('Problem In saving');
imagedestroy($dest);

// Connect to the database:
mysql_connect('localhost', 'db_username', 'db_pass')
or die('Problems connection to database server');
mysql_select_db('db_name')
or die('Couldnt use selected Database. ' . mysql_error());

mysql_query("INSERT INTO filenames(filename, description) VALUES('$userfile_name', '$userfile_description')")
or die('Error in SQL: ' . mysql_error());

mysql_close();
}
echo '
<a href="' . $prod_img . '">
<img src="' . $prod_img_thumb . '" width="' . $new_w . '" heigt="' . $new_h . '" alt="" />
</a>';
// show an upload form to upload and resize an image
}
else
{
echo '
<form method="post" action="' . $_SERVER['PHP_SELF'] . '" enctype="multipart/form-data">
<input type="file" name="image" />
<input type="text" name="description" />
<input type="submit" name="submit" value="upload and resize image" />
</form>';
}
?>