$im = imagecreatefromgif($giffile);

if ($im === false) {
    echo "$giffile is not GIF file\n";
    exit (1);
}

$colormap_num = imagecolorstotal($im);
$transparent_index = imagecolortransparent($im);

$colormap = '';

if ($transparent_index < 0) {
    for ($i = 0 ; $i < $colormap_num ; $i++) {
        $rgb = imagecolorsforindex($im, $i);
        $colormap .= chr($rgb['red']);
        $colormap .= chr($rgb['green']);
        $colormap .= chr($rgb['blue']);
    }
} else {
    for ($i = 0 ; $i < $colormap_num ; $i++) {
        $rgb = imagecolorsforindex($im, $i);
        $colormap .= chr($rgb['red']);
        $colormap .= chr($rgb['green']);
        $colormap .= chr($rgb['blue']);
	$colormap .= ($i == $transparent_index)?chr(0):chr(255);
    }
}

$indices = '';
$i = 0;
$width  = imagesx($im);
$height = imagesy($im);

for ($y = 0 ; $y < $height ; $y++) {
    for ($x = 0 ; $x < $width ; $x++) {
        $indices .= chr(imagecolorat($im, $x, $y));
        $i++;
    }
    while (($i % 4) != 0) {
        $indices .= chr(0);
        $i++;
    }
}

$format = chr(3); // palett format
$content = pack('v', $image_id).$format.pack('v', $width).pack('v', $height);
$content .= chr($colormap_num - 1).gzcompress($colormap.$indices);

if ($transparent_index < 0) {
    $tagCode = 20; // DefineBitsLossless
} else {
    $tagCode = 36; // DefineBitsLossless2
}