imagepalettecopy

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: 
<?
// this is a drop-in replacement for imagepalettecopy, except that it make NO attempt to modifiy any of the 
// colors in the dest image, just the palette.   The result?  if you're palette's aren't very similar, the image will look completely different, and likely terrible!
function imagepalettecopy_exact ( $dst_img, $src_img) {
    for( $c = 0 ; $c < imagecolorstotal($src_img); $c++) {
        $col = imagecolorsforindex($src_img,$c);  //get color at index 'c' in the color table
        imagecolorset($dst_img,$c,$col[red],$col[green],$col[blue]); //set color at index 'c' to $col in the $dst_image
    }
}

?>

To be precise, this function replaces the palette in the destination.