Usage example:
<code lang="PHP">

<?php

include('class.upload.php');

// Create the upload object
$uploader = new swd_upload;

// Set an upload limit
$uploader->max_size *= 15; // 15 MB

// Add some extensions
$uploader->add_extension('jpg', 'bmp', 'png');

// Set an upload dir
$uploader->upload_path = './uploads/';

// Upload file(s)
$uploader->upload();

// Show the upload form. Use display_transloader() for the transloader.
$uploader->display_uploader('upload.php', 20);

// Show allowed extensions to user
echo $uploader->get_extensions(true);

// Display all uploaded files
$uploader->get_uploaded_files();

// Display all errors
$uploader->get_errors();

?> 

</code>

Note that most of the steps can be skipped if they're already configured in the class itself. Check the first variables.

Here the class:

<code lang="PHP">

<?php

class swd_upload
{
	/**
	*	=========================================================================
	*								UPLOAD CLASS
	*	=========================================================================
	*
	*	@author:			Nicolas Oelgart
	*	@copyright:			Nicolas Oelgart
	*	@version:			Revision: 1.8
	*	@date:				25th August 2006
	*	@last modified:		16th October 2006
	*	@license:			GPL
	*
	*	@website:			www.nicoswd.com
	*	@email:				nico@nicoswd.com
	*
	*	@requirements:		PHP 4.2 +
	*
	*	=========================================================================
	*
	*	Copyright (C) 2006  Nicolas Oelgart
	*	
	*	This program is free software; you can redistribute it and/or modify it
	*	under the terms of the GNU General Public License as published by the
	*	Free Software Foundation; either version 2 of the License, or (at your
	*	option) any later version.
	*	
	*	This program is distributed in the hope that it will be useful, but
	*	WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
	*	or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
	*	for more details.
	*	
	*	You should have received a copy of the GNU General Public License along 
	*	with this program; if not, write to the Free Software Foundation, Inc., 
	*	51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
	*
	*	=========================================================================
	*/

	/**
	*	Max upload size in bytes. Set to FALSE fot no limit.
	*	Note: 1048576 byes = 1 MB
	*
	*	@var	integer
	*/
	var $max_size = 1048576;

	/**
	*	Allowed uploads at the same time.
	*
	*	@var	integer
	*/
	var $max_uploads = 5;

	/**
	*	Default allowed file extensions. (Lowercase)
	*
	*	@var	array
	*/
	var $extensions = array('jpg', 'zip', 'pdf');

	/**
	*	Upload path. Will be created if not exists.
	*
	*	@var	string
	*/
	var $upload_path = './uploads/';

	/**
	*	Will create subdirs based on the date format in the next variable.
	*
	*	@var	boolean
	*/
	var $enable_subdirs = true;
	
	/**
	*	Subdir format. Must be a valid format for PHP's date() function.
	*	See first table on this page: http://us2.php.net/date
	*
	*	@var	string
	*/
	var $subdir_format = 'Ym';

	/**
	*	Creates blank index files in each upload and subdirectory if TRUE.
	*
	*	@var	boolean
	*/
	var $create_index = true;

	/**
	*	Filename for index files.
	*
	*	@var	string
	*/
	var $index_file = 'index.html';

	/**
	*	Overwrite existing files. Will create an unique filename if FALSE.
	*
	*	@var	boolean
	*/
	var $overwrite = false;

	/**
	*	All error messages are here. You may wish to rephrase them.
	*
	*	@var	array
	*/
	var $default_errors = array
	(
		// Default errors
		'',
		'The uploaded file exceeds the upload_max_filesize directive.',
		'The uploaded file exceeds the MAX_FILE_SIZE directive that was specified in the HTML form.',
		'The uploaded file was only partially uploaded.',
		'No file was uploaded.',
		'Missing a temporary folder.',
		'Failed to write file to disk.',
		'Unknown File Error.',
		
		// Custom errors
		'Extension (%s) not allowed. "%s" not uploaded.',
		'File over allowed size of %d bytes.',
		'The upload of "%s" failed. Please try again.',
		'Max uploads of %d has been reached or an unexpected error ocurred.',
		'An unexpected error ocurred. %s has not been uploaded.',
		'Could not create upload directory. Upload cancelled.',
		'Failed to open file. Please check the URL.',
		'The server has disabled this function. Please contact the administrator.',
		'The upload directory is not writable. Could not upload file.'
	);

	/**
	*	====================================================================================
	*									END CONFIGURATION.
	*	====================================================================================
	*
	*	You may not edit below unless you know what you're doing.
	*/


	/**
	*	Holds all errors that ocurred while uplading.
	*
	*	@var	array
	*/
	var $errors = array();

	/**
	*	Holds all names of all uploaded files.
	*
	*	@var	array
	*/	
	var $uploaded_files = array();

	/**
	*	Holds the formatted subdir name if enabled.
	*
	*	@var	string
	*/	
	var $subdir = '';

	/**
	*	Holds the $_FILES['swd_upload'] array.
	*
	*	@var	array
	*/
	var $files;

	/**
	*	Holds the $_POST['swd_transload'] file URL.
	*
	*	@var	string
	*/
	var $transload;

	/**
	*	Holds the current key.
	*
	*	@var	integer
	*/
	var $_key;

	
	
	/**
	*	Class constructor
	*
	*	Sets main variables.
	*/	
	function swd_upload()
	{
		$this->files = $_FILES['swd_upload'];
		$this->transload = $_POST['swd_transload'];
		
		if ($this->enable_subdirs)
		{
			$this->subdir = date($this->subdir_format) .'/';
		}
	}
	
	
	/**
	*	Checks quick if the filename is valid, and if the number of max uploads
	*	hasn't been passed yet. Sends array key to move() if everything ok here.
	*
	*	@return		mixed
	*/
	function upload()
	{
		if (!is_array($this->files['error']) OR !$this->is_valid_upload_path())
		{
			return false;
		}
		
		foreach ($this->files['error'] AS $key => $error)
		{
			$filename = $this->files['name'][$key];
			$this->_key = $key;
			
			if (empty($filename))
			{
				continue;
			}
			else if (sizeof($this->uploaded_files) >= $this->max_uploads)
			{
				$this->set_error(sprintf($this->default_errors[11], $this->max_uploads));
				return false;
			}
			else if ($this->is_valid_file($filename) AND ($error === 0))
			{
				$this->move();
			}
			else if (!empty($error))
			{
				$this->set_error($this->default_errors[$error]);
			}
		}
	}
	
	
	/**
	*	Transloads a file from another host to the upload directory.
	*
	*	@return		boolean
	*/
	function transload()
	{
		if (ini_get('allow_url_fopen') == 0)
		{
			$this->set_error($this->default_errors[15]);
			return false;
		}
	
		if (!$this->is_valid_upload_path())
		{
			return false;
		}
	 
		if (!empty($this->transload))
		{
			$filename = basename($this->transload);
			$contents = '';
			
			if (!$this->is_valid_file($filename, false))
			{
				return false;
			}

			set_time_limit(0);
			@ini_set('user_agent', 'PHP');

			if (!($fp = @fopen($this->transload, 'rb')))
			{
				$this->set_error($this->default_errors[14]);
				return false;
			}
			
			while (!feof($fp))
			{
				$contents .= fread($fp, 8192);
				
				if ($this->max_size AND (strlen($contents) > $this->max_size))
				{
					$this->set_error(sprintf($this->default_errors[9], $this->max_size));
					return false;
				}
			}
			
			fclose($fp);
	
			if ($upload = @fopen($this->upload_path . $this->subdir . $this->rand_name($filename), 'w'))
			{
				fwrite($upload, $contents);
				fclose($upload);
			}
			
			$this->uploaded_files[] = $filename;
			return true;
		}
	}
		
	
	/**
	*	Checks if the file has a valid extension and if the size is not over the 
	*	max upload limit.
	*
	*	@param		string		Filename
	*	@param		boolean		If true, checks for size as well.
	*
	*	@return		boolean
	*/
	function is_valid_file($filename, $checksize = true)
	{
		if (!in_array($this->file_extension($filename), $this->extensions))
		{
			$this->set_error(sprintf($this->default_errors[8], $this->file_extension($filename), $filename));
			return false;
		}
		
		if ($checksize AND $this->max_size)
		{
			if ($this->files['size'][$this->_key] > $this->max_size)
			{
				$this->set_error(sprintf($this->default_errors[9], $this->max_size));
				return false;
			}
		}
		
		return true;
	}
	
	
	/**
	*	Moves the uploaded file from the temp directory to the upload directory.
	*/
	function move()
	{
		$filename = $this->files['name'][$this->_key];
		
		if (is_uploaded_file($this->files['tmp_name'][$this->_key]))
		{
			$upload_name = $this->rand_name($filename);
			
			if (@move_uploaded_file($this->files['tmp_name'][$this->_key], $this->upload_path . $this->subdir . $upload_name))
			{
				array_push($this->uploaded_files, $upload_name);
			}
			else
			{
				$this->set_error(sprintf($this->default_errors[10], $filename));
			}
		}
		else
		{
			$this->set_error(sprintf($this->default_errors[12], $filename));
		}
	}
	
	
	/**
	*	Checks if the upload path (and subdir if enabled) exists and attempts 
	*	to create it if not. Also creates empty index files if enabled.
	*
	*	@return		boolean
	*/
	function is_valid_upload_path()
	{
		$paths = array($this->upload_path);
		
		if ($this->enable_subdirs)
		{
			array_push($paths, $this->upload_path . $this->subdir);
		}
		
		foreach ($paths AS $path)
		{
			if (!file_exists($path) AND !@mkdir($path, 0755))
			{
				$this->set_error($this->default_errors[13]);
				return false;
			}
			else if (!is_writable($path) AND !chmod($path, 755))
			{
				$this->set_error($this->default_errors[17]);
				return false;
			}
			
			if ($this->create_index AND !file_exists($path . $this->index_file))
			{
				@touch($path . $this->index_file);
			}
		}
		
		return true;
	}


	/**
	*	Adds one or more extensions to the allowed extensions array.
	*
	*	@param		mixed		Allowed extensions. Can either be an array or strings as arguments.
	*
	*	@return		boolean
	*/	
	function add_extension()
	{
		$extensions = func_get_args();
		
		return $this->extensions = array_merge($this->extensions, (is_array($extensions[0]) ? $extensions[0] : $extensions));
	}
	
	
	/**
	*	Removes one or more extensions from the allowed extensions array.
	*
	*	@param		mixed		Extensions that need to be removed. Can either be an array or strings as arguments.
	*
	*	@return		boolean
	*/	
	function remove_extension()
	{
		$extensions = func_get_args();

		return $this->extensions = array_diff($this->extensions, (is_array($extensions[0]) ? $extensions[0] : $extensions));
	}
	
	
	/**
	*	Returns all allowed extensions either as array or string.
	*
	*	@param		boolean		Returns the extensions as array if FALSE. 
	*	@param		string		Separator between extensions.
	*
	*	@return		mixed
	*/		
	function get_extensions($as_string = false, $separator = ', ')
	{
		$extensions = array_unique($this->extensions);
		
		return ($as_string ? implode($separator, $extensions) : $extensions);
	}
	
	
	/**
	*	Returns the last characters after the last dot in the given string.
	*
	*	@param		string		Filename
	*
	*	@return		string		Extension
	*/
	function file_extension($filename)
	{
		return strtolower(substr(strrchr($filename, '.'), 1));
	}
	

	/**
	*	Generates a unique name for the uploaded file if enabled.
	*
	*	@param		string		Filename
	*
	*	@return		string		Random name
	*/
	function rand_name($original)
	{
		if ($this->overwrite OR !file_exists($this->upload_path . $this->subdir . $original))
		{
			return $original;
		}
		else
		{
			return $this->rand_name(substr(md5(rand(1, 5000) . time() . rand(50, 7000)), -5) .'_'. $original);
		}
	}
	
	
	/**
	*	Adds an error to the array.
	*
	*	@param		string		Error message
	*
	*	@return		none
	*/
	function set_error($error)
	{
		$this->errors[] = $error;
	}
		
	
	/**
	*	Displays the upload form based on the allowed max uploads.
	*
	*	@param		string		If the form should be submitted to another file, this needs to be changed to the target filename.
	*
	*	@return		string		Upload form
	*/
	function display_uploader($action = false)
	{
		echo '<div id="swd_upload">'. "\r\n";
		echo '<form action="'. ($action ? $action : basename($_SERVER['PHP_SELF'])) .'" method="post" enctype="multipart/form-data">'. "\r\n";
		
		for ($i = 0; $i < $this->max_uploads; $i++)
		{
			echo "\t" .'<p><input type="file" name="swd_upload[]" /></p>'. "\r\n";
		}
		
		echo "\t" .'<input type="submit" value="Upload files" />'. "\r\n";
		echo '</form>'. "\r\n";
		echo '</div>'. "\r\n";
	}
	
	
	/**
	*	Displays the transloader form.
	*
	*	@param		mixed		Target filename for the form, or FALSE for self.
	*	@param		integer		Size for the text field.
	*
	*	@return		string		Upload form.
	*/	
	function display_transloader($action = false, $size = 20)
	{
		echo '<div id="swd_transload">'. "\r\n";
		echo '<form action="'. ($action ? $action : basename($_SERVER['PHP_SELF'])) .'" method="post">'. "\r\n";
		echo "\t". '<input type="text" size="'. $size .'" name="swd_transload" />'. "\r\n";
		echo "\t". '<input type="submit" value="Transload" />'. "\r\n";
		echo '</form>' ."\r\n";
		echo '</div>'. "\r\n";
	}
	
	
	/**
	*	Returns all errors either as array or as HTML list.
	*
	*	@param		boolean		If true, returns errors as array.
	*
	*	@return		mixed
	*/
	function get_errors($as_array = false)
	{
		if ($as_array OR sizeof($this->errors) === 0)
		{
			return ($as_array ? $this->errors : '');
		}
		
		echo '<div id="swd_errors">'. "\r\n";
		echo '<ul>' ."\r\n";
		
		foreach ($this->errors AS $error)
		{
			echo "\t". '<li>'. $error .'</li>'. "\r\n";
		}
		
		echo '</ul>' ."\r\n";
		echo '</div>' ."\r\n";
	}
	
	/**
	*	=========================================================================
	*								UPLOAD CLASS END
	*	=========================================================================
	*	Copyright (C) 2006 by Nicolas Oelgart.
	*/

}

?>

</code>