actually it doesn't "copy" the palette exactly. It copys the colors from the source palette to the destination image. the palette you end up with in the destination image will be "same colors different order". If you want an EXACT palette copy (at the expense of messing up your image if you aren't careful), then use this code:
<?
function imagepalettecopy_exact ( $dst_img, $src_img) {
for( $c = 0 ; $c < imagecolorstotal($src_img); $c++) {
$col = imagecolorsforindex($src_img,$c); imagecolorset($dst_img,$c,$col[red],$col[green],$col[blue]); }
}
?>