Some hint if you want to use this code for true-color alpha channel transparent images (actually tried it on png-s).
After the

$new_img = imagecreatetruecolor($neww,$newh);

line I added these:

imagealphablending( $new_img, false );
$col = imagecolorallocatealpha( $new_img, 0, 0, 0, 127 );
imagefilledrectangle( $new_img, 0, 0, $neww, $newh, $col );
imagealphablending( $new_img, true );
imageSaveAlpha($new_img, true);

This fills the new image with fully transparent pixels.
The most painful was the last function call, because it took some half an hour to discover that without it the alpha channel isn\'t saved with the image.