After some INet searches and personal try-and-failures I succeed to rotate PNG images with preserving alpha channel transparency (semi transparency).
<?php
$filename = 'YourFile.png';
$rotang = 20; $source = imagecreatefrompng($filename) or die('Error opening file '.$filename);
imagealphablending($source, false);
imagesavealpha($source, true);
$rotation = imagerotate($source, $rotang, imageColorAllocateAlpha($source, 0, 0, 0, 127));
imagealphablending($rotation, false);
imagesavealpha($rotation, true);
header('Content-type: image/png');
imagepng($rotation);
imagedestroy($source);
imagedestroy($rotation);
?>
