如何用PHP创建裁剪缩略图？

I'm trying here to create interface to load image and then select area of this image to save cropped thumbnail along with original image. To select cropping area I use jquery.Jcrop plugin and it works fine at the first gaze. Then I post relevant x-y width-height data to my php script and trying to save thumbnail with this lines of code:

$source = imagecreatefromjpeg(UPLOADED_IMAGE);
$target = imagecreatetruecolor(TWIDTH, THEIGHT);

imagecopyresampled($target,$source,0,0,$_POST['x'],$_POST['y'],
    TWIDTH,THEIGHT,$_POST['w'],$_POST['h']);

imagejpeg($target, FINAL_THUMBNAIL_DIRECTORY.$targetFile, 100);

to ensure that all data is correct:

$imageSize =  getimagesize(UPLOADED_IMAGE);
echo UPLOADED_IMAGE . "
";
echo $imageSize[0] . "x" . $imageSize[1] . "
";
echo TWIDTH . "x" . THEIGHT . "
";
echo $_POST['x'] . "x" . $_POST['y'] . " - " . $_POST['w'] . "x" . $_POST['h'] . "
";
echo FINAL_THUMBNAIL_DIRECTORY.$targetFile . "
";
exit();

as you may see on this image (result of php script in alert window), all variables passed in my script is valid. UPLOADED_IMAGE is there, available by this link, but final saved image is 200x150 and solid black.

What may cause this problem? I'm running around this problem for a few days, although I'm not a php programmer, code looks quite simple