PhpThumb错误-PHP致命错误:Class'；PhpThumb'；找不到

我从官方网站下载，上传到我的托管服务器。我收到了这个错误消息。

[08-Mar-2012 21:26:07] PHP Fatal error: Class 'PhpThumb' not found in /home/sokad
/public_html/libs/phpthumb/thumb_plugins/gd_reflection.inc.php on line 179 
Fatal error: Uncaught exception 'RuntimeException' with message 'Cannot show image, headers 
have already been sent' in /home/sokad/public_html/libs/phpthumb/GdThumb.inc.php:569 Stack 
trace: #0 /home/sokad/public_html/resize.php(14): GdThumb->show() #1 {main} thrown in 
/home/sokad/public_html/libs/phpthumb/GdThumb.inc.php on line 569
require_once 'libs/phpthumb/ThumbLib.inc.php';
$thumb = PhpThumbFactory::create($url);
$thumb->adaptiveResize($width, $height);
$thumb->show();

我发现很多网站也出现了同样的问题。知道怎么解决吗？