Turning this upload script from "1 file upload capable" to "multi-file upload capable"

=========================================== upload.php:
 
<!-- ******************************************************
HTML FORM SECTION:
*********************************************************** -->
 
<form action="<?php echo $_SERVER['PHP_SELF']; ?>" method="post" enctype="multipart/form-data">
   <p>
      <label for="file">Select a file:</label> 
      <input type="file" name="userfile" id="file"><br />
      <input type="submit" value="Submit" />
   <p>
</form>
 
 
<!-- ******************************************************
UPLOAD SECTION:
*********************************************************** -->
 
<?php
      $allowed_filetypes = array('.jpg','.gif','.bmp','.png'); 					
      $max_filesize = 524288; 																
      $upload_path = './files/'; 															
 
   	$filename = $_FILES['userfile']['name']; 											
   	$ext = substr($filename, strpos($filename,'.'), strlen($filename)-1); 
 
  		if(!in_array($ext,$allowed_filetypes))
     		die('The file you attempted to upload is not allowed.');
 
   	if(filesize($_FILES['userfile']['tmp_name']) > $max_filesize)
      	die('The file you attempted to upload is too large.');
 
   	if(!is_writable($upload_path))
      	die('You cannot upload to the specified directory, please CHMOD it to 777.');
   	
   	if(move_uploaded_file($_FILES['userfile']['tmp_name'],$upload_path . $filename))
         echo 'Your file upload was successful, view the file <a href="' . $upload_path . $filename . '" title="Your File">here</a>'; 
      else
         echo 'There was an error during the file upload.  Please try again.'; 
  
?>